vout_macosx: protect vout_display_SendEvent calls to prevent potential crashes

modules/video_output/macosx.m

@@ -701,32 +701,46 @@ static void OpenglSwap (vlc_gl_t *gl)
 
 - (void)mouseDown:(NSEvent *)o_event
 {
+    @synchronized (self) {
+        if (vd) {
             if ([o_event type] == NSLeftMouseDown && !([o_event modifierFlags] &  NSControlKeyMask)) {
                 if ([o_event clickCount] <= 1)
                     vout_display_SendEventMousePressed (vd, MOUSE_BUTTON_LEFT);
             }
+        }
+    }
 
     [super mouseDown:o_event];
 }
 
 - (void)otherMouseDown:(NSEvent *)o_event
 {
+    @synchronized (self) {
+        if (vd)
             vout_display_SendEventMousePressed (vd, MOUSE_BUTTON_CENTER);
+    }
 
     [super otherMouseDown: o_event];
 }
 
 - (void)mouseUp:(NSEvent *)o_event
 {
+    @synchronized (self) {
+        if (vd) {
             if ([o_event type] == NSLeftMouseUp)
                 vout_display_SendEventMouseReleased (vd, MOUSE_BUTTON_LEFT);
+        }
+    }
 
     [super mouseUp: o_event];
 }
 
 - (void)otherMouseUp:(NSEvent *)o_event
 {
+    @synchronized (self) {
+        if (vd)
             vout_display_SendEventMouseReleased (vd, MOUSE_BUTTON_CENTER);
+    }
 
     [super otherMouseUp: o_event];
 }