lavc/videotoolboxenc: return SEI parse errors

Return the actual find_sei_end() error when SEI appending fails instead of
reusing the previous status code. This preserves the real parse failure for
callers instead of reporting malformed SEI handling as success.

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavcodec/videotoolboxenc.c

@@ -2150,7 +2150,7 @@ static int copy_replace_length_codes(
             uint8_t *new_sei;
             old_sei_length = find_sei_end(avctx, dst_box, box_len, &new_sei);
             if (old_sei_length < 0)
-                return status;
+                return old_sei_length;
 
             wrote_bytes = write_sei(sei,
                                     SEI_TYPE_USER_DATA_REGISTERED_ITU_T_T35,