Return on a stalled non-blocking gzprintf write

[carrerasdarren-cell]

Summary

• return Z_BUF_ERROR immediately when a formatted gzip write stalls on a non-blocking destination
• apply the same control-flow fix to both gzvprintf() and the legacy C89 gzprintf() implementation
• preserve the existing retry contract instead of continuing to copy the same input into an exhausted output buffer

Fixes #1256.

Root cause

When pending gzip output cannot be flushed, both formatted-write paths record Z_BUF_ERROR but continue processing the same input. The write cursor can remain at the end of the allocation, allowing the following copy to write beyond the heap buffer.

Returning the recorded error at the stall point lets the caller wait for the descriptor to become writable and retry the same formatted write.

Validation

• reproduced the untouched heap-buffer-overflow with a deterministic non-blocking pipe proof under AddressSanitizer
• verified the patched sanitizer and release builds return Z_BUF_ERROR, successfully retry the same write after draining the pipe, and close with Z_OK
• make -j4 test
• make -j4 testshared