SimpleCache: handle null callback to ops we may need to defer, like Doom

Hoping to fix 881941

Bug: 881941
Change-Id: I7ba5b34b3e3f0c6eec8747a0079a95a1f44aa732
Reviewed-on: https://chromium-review.googlesource.com/1214302Reviewed-by: Bence Béky <bnc@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589934}

net/disk_cache/simple/simple_backend_impl.cc

@@ -166,10 +166,12 @@ base::RepeatingCallback<void(int)> MakeBarrierCompletionCallback(
 void RunOperationAndCallback(
     base::OnceCallback<int(net::CompletionOnceCallback)> operation,
     net::CompletionOnceCallback operation_callback) {
-  auto copyable_callback =
+  base::RepeatingCallback<void(int)> copyable_callback;
+  if (operation_callback)
+    copyable_callback =
         base::AdaptCallbackForRepeating(std::move(operation_callback));
   const int operation_result = std::move(operation).Run(copyable_callback);
-  if (operation_result != net::ERR_IO_PENDING)
+  if (operation_result != net::ERR_IO_PENDING && copyable_callback)
     copyable_callback.Run(operation_result);
 }