Addresses issue #6268 by identifying and partially fixing the root cause.

ROOT CAUSE DISCOVERED: Cloning Issue (similar to #6121)
- In match cases, HideRevealStmt objects are cloned during desugaring
- Cloning creates new AssertLabel objects without preserving E field
- Original AssertLabel objects get E field filled during Boogie generation
- Cloned AssertLabel objects accessed by reveal statements have null E field
- Contract.Assert(la.E != null) crashes when accessing cloned objects

THE FIX: Graceful Null Handling
- Skip processing AssertLabel objects with null E field
- Prevents crash but doesn't restore full functionality
- Reveal statements in match cases don't work but don't crash

LIMITATION: Functionality Not Fully Restored
- This prevents the crash but reveal statements don't work in match cases
- Full fix would require connecting cloned objects to original translated expressions
- Similar pattern to ForallStmt fix in #6121 but more complex due to E field timing

This demonstrates the cloning issue pattern and provides crash prevention
while acknowledging the need for a more complete architectural solution.