macosx: fixed potential race condition

(cherry picked from commit 5b8a40579cfbacb736484f4210f1cc93e9caf440)

modules/gui/macosx/intf.m

@@ -778,6 +778,7 @@ static VLCMain *_o_sharedMainInstance = nil;
 
     [o_msg_arr removeAllObjects];
     [o_msg_arr release];
+    o_msg_arr = NULL;
 
     [o_msg_lock release];
 
@@ -2000,6 +2001,8 @@ unsigned int CocoaKeyToVLC( unichar i_key )
 
 - (void)processReceivedlibvlcMessage:(const msg_item_t *) item ofType: (int)i_type withStr: (char *)str
 {
+    if (o_msg_arr)
+    {
         NSColor *o_white = [NSColor whiteColor];
         NSColor *o_red = [NSColor redColor];
         NSColor *o_yellow = [NSColor yellowColor];
@@ -2030,6 +2033,7 @@ unsigned int CocoaKeyToVLC( unichar i_key )
 
         b_msg_arr_changed = YES;
         [o_msg_lock unlock];
+    }
 }
 
 - (IBAction)saveDebugLog:(id)sender