smptealpha: Do not set width/height before comparing with old values

Otherwise we end up considering the values did not change and we wrongly
work with the old video format (which will lead to wrong
behaviour/segfaults).

https://bugzilla.gnome.org/show_bug.cgi?id=755621

gst/smpte/gstsmptealpha.c

@@ -615,22 +615,20 @@ gst_smpte_alpha_set_info (GstVideoFilter * vfilter, GstCaps * incaps,
 {
   GstSMPTEAlpha *smpte = GST_SMPTE_ALPHA (vfilter);
   gboolean ret;
-  gint width, height;
 
   smpte->process = NULL;
-
   smpte->in_format = GST_VIDEO_INFO_FORMAT (in_info);
   smpte->out_format = GST_VIDEO_INFO_FORMAT (out_info);
-  smpte->width = width = GST_VIDEO_INFO_WIDTH (out_info);
-  smpte->height = height = GST_VIDEO_INFO_HEIGHT (out_info);
 
   /* try to update the mask now, this will also adjust the width/height on
    * success */
   GST_OBJECT_LOCK (smpte);
   ret =
       gst_smpte_alpha_update_mask (smpte, smpte->type, smpte->invert,
-      smpte->depth, width, height);
+      smpte->depth, GST_VIDEO_INFO_WIDTH (out_info),
+      GST_VIDEO_INFO_HEIGHT (out_info));
   GST_OBJECT_UNLOCK (smpte);
+
   if (!ret)
     goto mask_failed;