avidemux: rearrange some checks to avoid NULL use

gst/avi/gstavidemux.c

@@ -4468,6 +4468,11 @@ gst_avi_demux_invert (GstAviStream * stream, GstBuffer * buf)
   if (stream->strh->type != GST_RIFF_FCC_vids)
     return buf;
 
+  if (stream->strf.vids == NULL) {
+    GST_WARNING ("Failed to retrieve vids for stream");
+    return buf;
+  }
+
   fourcc = (stream->strf.vids->compression) ?
       stream->strf.vids->compression : stream->strh->fcc_handler;
   if (!gst_avi_demux_is_uncompressed (fourcc)) {
@@ -4483,11 +4488,6 @@ gst_avi_demux_invert (GstAviStream * stream, GstBuffer * buf)
     return buf;
   }
 
-  if (stream->strf.vids == NULL) {
-    GST_WARNING ("Failed to retrieve vids for stream");
-    return buf;
-  }
-
   h = stream->strf.vids->height;
   w = stream->strf.vids->width;
   stride = GST_ROUND_UP_4 (w * (bpp / 8));