Fixed a crash that happened when changing the rewind length in Cocoa

Cocoa/Document.m

@@ -1411,9 +1411,11 @@ static void printImage(GB_gameboy_t *gb, uint32_t *image, uint8_t height,
 
 - (void) updateRewindLength
 {
+    [self performAtomicBlock:^{
         if (GB_is_inited(&gb)) {
             GB_set_rewind_length(&gb, [[NSUserDefaults standardUserDefaults] integerForKey:@"GBRewindLength"]);
         }
+    }];
 }
 
 @end