[mojo] Check that shared memory is valid before trying to write into it

It's possible for BigBufferView::shared_memory_ to be null if memory
mapping fails. This change adds a check that the mapping is not null
before trying to write there.

Bug: 961256
Change-Id: I72772e07583ab62df5544d419604356071071e94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611311Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660010}

mojo/public/cpp/base/big_buffer.cc

@@ -112,9 +112,11 @@ BigBufferView::BigBufferView(base::span<const uint8_t> bytes) {
     if (buffer.is_valid()) {
       storage_type_ = BigBuffer::StorageType::kSharedMemory;
       shared_memory_.emplace(std::move(buffer), bytes.size());
-      std::copy(bytes.begin(), bytes.end(),
-                static_cast<uint8_t*>(shared_memory_->buffer_mapping_.get()));
-      return;
+      if (shared_memory_->buffer_mapping_) {
+        std::copy(bytes.begin(), bytes.end(),
+                  static_cast<uint8_t*>(shared_memory_->buffer_mapping_.get()));
+        return;
+      }
     }
 
     if (bytes.size() > kMaxFallbackInlineBytes) {