Fixed bug 3880 - X Error upon quit since rev. 11607

Ozkan Sezer

Since changeset 11607:60cd425a2f14, I am getting the following
error upon quit.  Running testsprite2, clicking the mouse, and
quiting it is enough to trigger it.  This is on my old Fedora9
x86-Linux:

X Error of failed request:  BadCursor (invalid Cursor parameter)
  Major opcode of failed request:  2 (X_ChangeWindowAttributes)
  Resource id in failed request:  0xb057340
  Serial number of failed request:  905
  Current serial number in output stream:  906

Reverting  https://hg.libsdl.org/SDL/rev/60cd425a2f14  removes
the error.

src/events/SDL_mouse.c

@@ -580,13 +580,16 @@ SDL_MouseQuit(void)
         SDL_FreeCursor(cursor);
         cursor = next;
     }
+    mouse->cursors = NULL;
 
     if (mouse->def_cursor && mouse->FreeCursor) {
         mouse->FreeCursor(mouse->def_cursor);
+        mouse->def_cursor = NULL;
     }
 
     if (mouse->clickstate) {
         SDL_free(mouse->clickstate);
+        mouse->clickstate = NULL;
     }
 
     SDL_DelHintCallback(SDL_HINT_MOUSE_NORMAL_SPEED_SCALE,