Do not enter testing mode if a valid mode is not selected.

- It doesn't make much sense to "test" the current mode if a non-video
  mode is selected in the menu, so don't.

src/menu/videomenu.cpp

@@ -166,6 +166,8 @@ public:
 				NewWidth = SCREENWIDTH;
 				NewHeight = SCREENHEIGHT;
 			}
+			else
+			{
 				OldWidth = SCREENWIDTH;
 				OldHeight = SCREENHEIGHT;
 				OldBits = DisplayBits;
@@ -176,6 +178,7 @@ public:
 				SetModesMenu (NewWidth, NewHeight, NewBits);
 				return true;
 			}
+		}
 		return Super::Responder(ev);
 	}
 };