Avoid overflow after a stalled gzprintf write.

[samuelgirmametaferia]

This fixes an overflow in the buffered gzwrite() path after a non-blocking gzprintf() stalls with more than state->size bytes pending in the double-sized input buffer.

Previously, the small-write path computed state->size - have without first handling have >= state->size, so the subtraction could underflow and lead to a copy past the input allocation. The fix first attempts to drain the pending input with gz_comp() and returns the non-blocking stall without consuming caller input when the output cannot make progress.

A focused regression test fills a non-blocking pipe, uses a small gzip buffer, performs two gzprintf() calls to reach the stalled state, and verifies that a following gzwrite() consumes no input.

Validation performed locally:

• static CMake build passes;
• all 14 CTest tests pass, including the new zlib_gzprintfwrite test;
• the regression test fails against the unpatched base;
• ASan reproduces the heap-buffer-overflow before the fix and no overflow occurs on the corrected path.

[samuelgirmametaferia]

Hey could you please review this!