Set NSFileManager delegate back to nil once copying is done.

Otherwise the next operation by NSFileManager will try to use the deallocated instance and crash.

MunkiAdmin/NSOperation Subclasses/MAFileCopyOperation.m

@@ -63,7 +63,7 @@ -(void)main {
             } else {
                 DDLogError(@"%@: Copy failed with error: %@", filename, [copyError description]);
             }
-
+            [fm setDelegate:nil];
 		}
 	}
 	@catch(...) {