vtenc: Set profile_level to NULL after release

Otherwise, gst_vtenc_negotiate_profile_and_level will double-release as
it checks for profile_level != NULL. This caused crashes when the
vtenc instance is stopped and then restarted.

https://bugzilla.gnome.org/show_bug.cgi?id=757935

sys/applemedia/vtenc.c

@@ -505,6 +505,7 @@ gst_vtenc_stop (GstVideoEncoder * enc)
 
   if (self->profile_level)
     CFRelease (self->profile_level);
+  self->profile_level = NULL;
 
   if (self->input_state)
     gst_video_codec_state_unref (self->input_state);