Fixed test programs for joystick not exiting on events after first disconnect.

Exit was broken since the main loop extraction needed for Emscripten support
because the former local but now global variables were not reset correctly.

test/testjoystick.c

@@ -176,6 +176,8 @@ WatchJoystick(SDL_Joystick * joystick)
 {
     SDL_Window *window = NULL;
     const char *name = NULL;
+
+    retval = SDL_FALSE;
     done = SDL_FALSE;
 
     /* Create a window to display joystick axis position */
@@ -217,6 +219,7 @@ WatchJoystick(SDL_Joystick * joystick)
 #endif
 
     SDL_DestroyRenderer(screen);
+    screen = NULL;
     SDL_DestroyWindow(window);
     return retval;
 }