Fix null pointer

In some cases, the fds parameter of frame_data_sequence_find is invalid,
causing the software to crash, For example, this command
echo'{"req":"frame","bytes":"yes","proto":"yes","frame":"1" }'|sharkd-

epan/frame_data_sequence.c

@@ -183,7 +183,7 @@ frame_data_sequence_find(frame_data_sequence *fds, guint32 num)
   frame_data ***level2;
   frame_data ****level3;
 
-  if (num == 0) {
+  if (num == 0 || fds == NULL) {
     /* There is no frame number 0 */
     return NULL;
   }