Add null check for blob_data_ in CalculateSizeImpl

Bug: 925306
Change-Id: I8aa9504c6ce8472101dbadf5e9f391971e5de1a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519427Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824406}

storage/browser/blob/blob_reader.cc

@@ -338,6 +338,10 @@ BlobReader::Status BlobReader::CalculateSizeImpl(
   DCHECK(!total_size_calculated_);
   DCHECK(size_callback_.is_null());
 
+  if (!blob_data_) {
+    return ReportError(net::ERR_UNEXPECTED);
+  }
+
   net_error_ = net::OK;
   total_size_ = 0;
   const auto& items = blob_data_->items();