Initialize QuicBatchWriterBuffer's internal buffer.

Merge internal change: 221684051

R=rch@chromium.org

Change-Id: I124ca8f5573966b7c8df35b28793003b834620a4
Reviewed-on: https://chromium-review.googlesource.com/c/1340399Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bin Wu <wub@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608956}

net/third_party/quic/platform/impl/batch_writer/quic_batch_writer_buffer.cc

@@ -8,7 +8,9 @@
 
 namespace quic {
 
-QuicBatchWriterBuffer::QuicBatchWriterBuffer() {}
+QuicBatchWriterBuffer::QuicBatchWriterBuffer() {
+  memset(buffer_, 0, sizeof(buffer_));
+}
 
 QuicString QuicBatchWriterBuffer::DebugString() const {
   std::ostringstream os;