fix frame size option, patch by Limin Wang, lance lmwang gmail com

Originally committed as revision 8165 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffplay.c

@@ -2341,7 +2341,7 @@ static void event_loop(void)
 
 static void opt_frame_size(const char *arg)
 {
-    if (parse_image_size(&screen_width, &screen_height, arg) < 0) {
+    if (parse_image_size(&frame_width, &frame_height, arg) < 0) {
         fprintf(stderr, "Incorrect frame size\n");
         exit(1);
     }