fix undefined behavior in EOCD backward scan

## 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