Add missing null-check to BlobDataHandle::GetBlobStatus.

This method was dereferencing a weak pointer without first making sure
the pointer wasn't null, unlike every other method in the class. This
fixes that.

Bug: 790772
Change-Id: I980fa47a5afceae36d5843056ed03a88e863adce
Reviewed-on: https://chromium-review.googlesource.com/802762Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521297}

storage/browser/blob/blob_data_handle.cc

@@ -95,6 +95,9 @@ bool BlobDataHandle::IsBroken() const {
 }
 
 BlobStatus BlobDataHandle::GetBlobStatus() const {
+  DCHECK(io_task_runner_->RunsTasksInCurrentSequence());
+  if (!shared_->context_)
+    return BlobStatus::ERR_REFERENCED_BLOB_BROKEN;
   return shared_->context_->GetBlobStatus(shared_->uuid_);
 }