Segmentation fault fix when no video device is specified.

patch by Ramiro Polla % ramiro A lisha P ufsc P br %

Originally committed as revision 7445 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -3235,7 +3235,7 @@ static void prepare_grab(void)
     if (has_video) {
         AVInputFormat *fmt1;
 #warning FIXME: find a better interface
-        if(!strncmp(video_device,"x11:",4)) {
+        if(video_device&&!strncmp(video_device,"x11:",4)) {
             video_grab_format="x11grab";
         }
         fmt1 = av_find_input_format(video_grab_format);