Fix GH-16780: gzseek aborts on non seekable stream.

the stream flags is set to non seekable, thus we bail out early
in the process.

main/streams/streams.c

@@ -1321,6 +1321,11 @@ PHPAPI int _php_stream_seek(php_stream *stream, zend_off_t offset, int whence)
 		}
 	}
 
+	/* the stream is not properly seekable */
+	if ((stream->flags & PHP_STREAM_FLAG_NO_SEEK) || (!stream->readpos && stream->position == -1)) {
+		goto fail;
+	}
+
 	/* handle the case where we are in the buffer */
 	if ((stream->flags & PHP_STREAM_FLAG_NO_BUFFER) == 0) {
 		switch(whence) {
@@ -1393,6 +1398,7 @@ PHPAPI int _php_stream_seek(php_stream *stream, zend_off_t offset, int whence)
 		return 0;
 	}
 
+fail:
 	php_error_docref(NULL, E_WARNING, "Stream does not support seeking");
 
 	return -1;