[zlib] Fix minigzip_fuzzer: compression level never varies from default

[OwenSanzas]

Summary

minigzip_fuzzer.c encodes the compression level as a raw integer (0x00–0x09) instead of an ASCII digit ('0'–'9'). Since gzopen() parses the mode string looking for characters in the range '0'–'9' (0x30–0x39), control characters 0x00–0x09 never match, and the compression level always falls back to Z_DEFAULT_COMPRESSION (level 6). The fuzzer's intent to test all 10 compression levels is completely defeated.

Bug (line 468):

outmode[2] = data[0] % 10;    // produces 0x00–0x09, not '0'–'9'

Fix:

outmode[2] = '0' + data[0] % 10;

In zlib's gz_open(), the level is parsed as:

while (*plevel) {
    if (*plevel >= '0' && *plevel <= '9') {
        level = *plevel - '0';
        break;
    }
    plevel++;
}

Since 0x00–0x09 don't satisfy >= '0' (0x30), the loop never matches.

Also fixes two resource leaks on error paths:

• file_compress(): FILE *in leaked when gzopen() fails
• file_uncompress(): gzFile in leaked when fopen() fails

Evidence

Coverage comparison (ASan, 60-second runs from seed corpus):

Metric	Original	Fixed	Change
INITED edges (same seed corpus)	815	866	+6.3% (+51 edges)
Final edges (after 60s)	870	968	+11.3% (+98 edges)

The fixed version reaches 51 more edges just from replaying the same seed corpus, because different compression levels exercise different code paths in zlib's deflate implementation.

[github-actions]

OwenSanzas is a new contributor to projects/zlib. The PR must be approved by known contributors before it can be merged. The past contributors are: hunsche, evverx, serge-sans-paille, vitaliset, fanquake, conNULL, hkctkuy, jvoisin, inferno-chromium, cvediver, Dor1s, gcp1304 (unverified), ProgramMax (unverified), sebpop (unverified)