fix: prevent signed integer overflow in gzseek64 for SEEK_SET

[SongTonyLi]

Summary

Fixes #1222. Generally agree with the issue's suggested fix, and I independently verified the patch's validity.

gzseek64() unconditionally subtracts state->x.pos from the user-supplied offset when whence == SEEK_SET. If offset is INT64_MIN and the stream position is non-zero, this overflows the signed z_off64_t type — undefined behavior flagged by UBSan.

Since a negative absolute position is always invalid for SEEK_SET, this patch rejects it early (returns -1) before the subtraction, eliminating the overflow without changing observable semantics.

Fix

Add if (offset < 0) return -1; guard inside the SEEK_SET branch, before offset -= state->x.pos.

Verification

Tested with UBSan-instrumented build:

• Before: gzlib.c:389:16: runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'off64_t'
• After: No UBSan findings; clean exit

Reproducer:

gzFile gz = gzopen("any_file", "rb");
char buf[2]; gzread(gz, buf, 2);  // advance pos to 1
gzseek(gz, LONG_MIN, SEEK_SET);   // triggers overflow without fix
gzclose(gz);