Fix a bug where an unsigned calculation goes negative and causes very large memcpy() call.

media/blink/multibuffer_reader.cc

Index: media/blink/multibuffer_reader.cc
diff --git a/media/blink/multibuffer_reader.cc b/media/blink/multibuffer_reader.cc
index b9d07529efc61c01fd6dcbee7654b7276c6afd68..2a695ff6797f53078b417a48d5e221965dd35b95 100644
--- a/media/blink/multibuffer_reader.cc
+++ b/media/blink/multibuffer_reader.cc
@@ -106,6 +106,8 @@ int64_t MultiBufferReader::TryRead(uint8_t* data, int64_t len) {
     if (i->second->end_of_stream())
       break;
     size_t offset = p & ((1LL << multibuffer_->block_size_shift()) - 1);
+    if (offset > static_cast<size_t>(i->second->data_size()))
+      break;
     size_t tocopy =
         std::min<size_t>(len - bytes_read, i->second->data_size() - offset);
     memcpy(data, i->second->data() + offset, tocopy);