Restore hash_head != 0 checks

Restore hash_head != 0 checks

Commit bc5915e2dec7 ("Fixed unsigned integer overflow ASAN error when
hash_head > s->strstart.") removed hash_head != 0 checks in fast,
medium and slow deflate, because it improved performance [1].

Unfortunately, the attached test started failing after that.
Apparently, as the comments suggest, the code implicitly relies on
matches with the beginning of the window being skipped. So restore the
check.

[1] https://github.com/zlib-ng/zlib-ng/pull/772#issuecomment-710760300