macOS: Fix a crash when the video subsystem quits, if SDL_MAC_NO_SANDBOX was defined when SDL was compiled.

src/video/cocoa/SDL_cocoamousetap.m

@@ -237,6 +237,13 @@ Cocoa_QuitMouseEventTap(SDL_MouseData *driverdata)
     SDL_MouseEventTapData *tapdata = (SDL_MouseEventTapData*)driverdata->tapdata;
     int status;
 
+    if (tapdata == NULL) {
+        /* event tap was already cleaned up (possibly due to CGEventTapCreate
+         * returning null.)
+         */
+        return;
+    }
+
     /* Ensure that the runloop has been started first.
      * TODO: Move this to InitMouseEventTap, check for error conditions that can
      * happen in Cocoa_MouseTapThread, and fall back to the non-EventTap way of