Fixed error log for non-fatal inflate() return code Z_BUF_ERROR

This MR fixes logs of the kind Failed to uncompress gzip stream (-5)

Z_BUF_ERROR is a non-fatal return code of inflate() and indicates that input and output is empty (size 0).

We already have

	if (!srclen) {
		// special case to avoid decompress errors
		if (dc->sink)
			dc->sink(dc->context, "", 0);

		return 0;
	}

to avoid this case (srclen = 0).

However, during the

	do {
		strm->next_out = (unsigned char *) dst;
		strm->avail_out = sizeof(dst);

		status = inflate(strm, Z_SYNC_FLUSH);
		if ((status == Z_OK || status == Z_STREAM_END) && strm->avail_out < sizeof(dst)) {
			if (dc->sink)
				dc->sink(dc->context, dst, sizeof(dst) - strm->avail_out);
		}
	} while (status == Z_OK && !strm->avail_out);

loop, inflate() may encounter a sitation where all input has been read, but no output can be returned and thus will return Z_BUF_ERROR.

Approver's checklist:

• The author has submitted the FSF Copyright Assignment and is listed in AUTHORS
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code