gh-154709: Fix out-of-bounds access in dict reverse iterator

[BHUVANSH855]

Summary

Fix an out-of-bounds access in dictreviter_iter_lock_held() when a reverse dictionary iterator is used after the underlying dictionary is cleared and modified.

The fix adds bounds checks before indexing into the dictionary's storage for both split-table and combined-table dictionaries. If the iterator's position is no longer valid, it now exits cleanly instead of accessing invalid memory.

A regression test has also been added to verify that this scenario no longer results in a crash.

Testing

• Reproduced the original crash using the provided reproducer before the fix.
• Verified that the reproducer no longer crashes after the fix.
• Ran:
  • ./python -m test test_dict
  • ./python -m test test_gc
  • ./python -m test test_builtin
  • ./python -m test test_dict test_gc test_builtin

• Issue: reversed(dict) can read out of bounds and segfault (dictreviter_iter_lock_held) #154709