fix undefined behavior in EOCD backward scan

[Naveed8951]

Description

This PR removes undefined behavior in the End Of Central Directory (EOCD)
signature search used when opening ZIP archives.

The previous implementation scanned backward using a pointer-decrement
loop of the form p >= data; p -= 1. When the scan reached the start of
the buffer, the subsequent decrement constructed an out-of-bounds
pointer (data - 1), which is undefined behavior in C. Under hardened
builds or sanitizers, this can result in process aborts.

Changes

• Replaced the pointer-based backward scan with an index-based scan that
  never constructs pointers outside the allocated buffer
• Added conservative validation of the scan bounds to ensure last
  remains within the buffer

Impact

• Eliminates undefined behavior in an untrusted-input parsing path
• Prevents crashes under UBSan and hardened runtimes
• Preserves existing EOCD search semantics

Testing

• Existing regression coverage exercises the non-archive open path
• Recommended verification:
  • Build with -fsanitize=undefined
  • Run the open-nonarchive regression test to confirm no UB reports