Describe the bug

SUBSTR returns an empty string when end is less than start. However, if start is 0, the first character is returned.

To reproduce

> set test cat
OK
> substr test 1 -500
""
> substr test 0 -500
"c"

Expected behavior

If end < start, SUBSTR should return an empty string.

Additional information

Redis server v=7.0.8 sha=00000000:0 malloc=jemalloc-5.2.1 bits=64 build=8b9bd5cdb53a6549