Throw an error if there are conflicting bcast tracking prefixes.

…-overlap

checkPrefixCollisionsOrReply documents a boolean contract: 1 on no
collision, 0 on collision (with the error reply already sent). The
sole caller in networking.c relies on this:

    if (!checkPrefixCollisionsOrReply(c, prefix, numprefix)) {
        zfree(prefix);
        return;
    }

When a collision between two user-provided prefixes is detected in
the inner j-loop, the function returns the outer loop index i instead
of 0. At i=0 this happens to be falsy and the caller behaves correctly,
which hid the bug from casual testing. For any i>=1 the non-zero value
is truthy, the caller falls through to enableTracking(), and the
client receives both the error reply and +OK while the overlapping
prefixes get registered:

    > CLIENT TRACKING ON BCAST PREFIX BAZ PREFIX FOOBAR PREFIX FOO
    -ERR Prefix 'FOOBAR' overlaps with another provided prefix 'FOO'...
    +OK

History
-------
The bug has been present since the function was introduced in redis#8176,
which added both the function and the boolean-style caller in the
same commit. The original contract was three-valued (-1 success,
0 existing-collision, i self-collision) but the caller used !retval,
so at i>=1 the self-collision path was silently a pass.

redis#8456 later changed the success return from -1 to 1 and tightened the
docstring to a boolean contract, but did not touch the `return i`
path below. That cleanup codified the boolean contract while leaving
the non-boolean exit alive, making the inherited bug easier to spot
by reading the function but no easier to catch at runtime.

Nothing in the tree consumes the returned index value -- it has
always been dead information. This change returns 0 in that path so
both error exits match the documented contract.

Test
----
Adds tests/unit/tracking.tcl case that triggers a self-collision at
i>=1 (BAZ FOOBAR FOO), asserts the error reply, and asserts
CLIENT TRACKINGINFO reports `flags off` -- i.e. tracking was not
wrongly enabled. Verified the test fails on master without the fix
and passes with it. Full unit/tracking suite green.

Signed-off-by: Raj Danday <rajkripal.danday@gmail.com>

…-overlap

Caller is `if (!checkPrefixCollisionsOrReply(...))` — expects 0 on
collision, non-zero on success. The self-collision branch returns the
outer loop index `i` instead of 0. At i=0 it's falsy so the caller bails
correctly. At i>=1 it's truthy, caller proceeds to enableTracking(),
client gets both the error reply AND +OK, overlapping prefixes get
registered.

Repro:
  CLIENT TRACKING ON BCAST PREFIX BAZ PREFIX FOOBAR PREFIX FOO
  -ERR Prefix 'FOOBAR' overlaps with 'FOO'
  +OK

Been there since redis#8176. redis#8456 flipped the success return to match a
boolean docstring but left `return i` alone. The index value isn't
read anywhere — always dead info.

Return 0 and add a regression test.

Signed-off-by: Raj Danday <rajkripal.danday@gmail.com>

Caller is `if (!checkPrefixCollisionsOrReply(...))` — expects 0 on
collision, non-zero on success. The self-collision branch returns the
outer loop index `i` instead of 0. At i=0 it's falsy so the caller bails
correctly. At i>=1 it's truthy, caller proceeds to enableTracking(),
client gets both the error reply AND +OK, overlapping prefixes get
registered.

Repro:
  CLIENT TRACKING ON BCAST PREFIX BAZ PREFIX FOOBAR PREFIX FOO
  -ERR Prefix 'FOOBAR' overlaps with 'FOO'
  +OK

Been there since redis#8176. redis#8456 flipped the success return to match a
boolean docstring but left `return i` alone. The index value isn't
read anywhere — always dead info.

Return 0 and add a regression test.

Signed-off-by: Raj Danday <rajkripal.danday@gmail.com>

Caller is `if (!checkPrefixCollisionsOrReply(...))` — expects 0 on
collision, non-zero on success. The self-collision branch returns the
outer loop index `i` instead of 0. At i=0 it's falsy so the caller bails
correctly. At i>=1 it's truthy, caller proceeds to enableTracking(),
client gets both the error reply AND +OK, overlapping prefixes get
registered.

Repro:
  CLIENT TRACKING ON BCAST PREFIX BAZ PREFIX FOOBAR PREFIX FOO
  -ERR Prefix 'FOOBAR' overlaps with 'FOO'
  +OK

Been there since redis#8176. redis#8456 flipped the success return to match a
boolean docstring but left `return i` alone. The index value isn't
read anywhere — always dead info.

Return 0 and add a regression test.

Signed-off-by: Raj Danday <rajkripal.danday@gmail.com>

Caller is `if (!checkPrefixCollisionsOrReply(...))` — expects 0 on
collision, non-zero on success. The self-collision branch returns the
outer loop index `i` instead of 0. At i=0 it's falsy so the caller bails
correctly. At i>=1 it's truthy, caller proceeds to enableTracking(),
client gets both the error reply AND +OK, overlapping prefixes get
registered.

Repro:
  CLIENT TRACKING ON BCAST PREFIX BAZ PREFIX FOOBAR PREFIX FOO
  -ERR Prefix 'FOOBAR' overlaps with 'FOO'
  +OK

Been there since redis#8176. redis#8456 flipped the success return to match a
boolean docstring but left `return i` alone. The index value isn't
read anywhere — always dead info.

Return 0 and add a regression test.

Signed-off-by: Raj Danday <rajkripal.danday@gmail.com>