Emscripten: Fixed receiving joystick events after failed init or subsystem quit.

The callbacks used to receive the HTML events were not removed if the joystick
subsystem initialization failed or if the joystick subsystem was quit. Also, the
already connected joysticks were not deleted if the initialization failed later.

src/joystick/emscripten/SDL_sysjoystick.c

@@ -220,6 +220,7 @@ SDL_SYS_JoystickInit(void)
                                                       Emscripten_JoyStickConnected);
 
     if(retval != EMSCRIPTEN_RESULT_SUCCESS) {
+        SDL_SYS_JoystickQuit();
         return -1;
     }
 
@@ -227,6 +228,7 @@ SDL_SYS_JoystickInit(void)
                                                          0,
                                                          Emscripten_JoyStickDisconnected);
     if(retval != EMSCRIPTEN_RESULT_SUCCESS) {
+        SDL_SYS_JoystickQuit();
         return -1;
     }
 
@@ -399,6 +401,9 @@ SDL_SYS_JoystickQuit(void)
 
     numjoysticks = 0;
     instance_counter = 0;
+
+    emscripten_set_gamepadconnected_callback(NULL, 0, NULL);
+    emscripten_set_gamepaddisconnected_callback(NULL, 0, NULL);
 }
 
 SDL_JoystickGUID SDL_SYS_JoystickGetDeviceGUID(int index)