Bug 1453134 - part 3 - be smarter about input stream semantics in DataTransfer; r=baku

We could have used the new NS_NewCStringInputStream overload here, but
it seemed nicer to directly transfer ownership into the newly-created
stream.  If we're going to be more efficient here, we might as well go
as far as when can without making the code too ugly.

dom/events/DataTransfer.cpp

@@ -6,6 +6,7 @@
 
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/BasicEvents.h"
+#include "mozilla/CheckedInt.h"
 
 #include "DataTransfer.h"
 
@@ -1470,11 +1471,14 @@ DataTransfer::FillInExternalCustomTypes(nsIVariant* aData, uint32_t aIndex,
     return;
   }
 
-  nsAutoCString str;
-  str.Adopt(chrs, len);
+  CheckedInt<int32_t> checkedLen(len);
+  if (!checkedLen.isValid()) {
+    return;
+  }
 
   nsCOMPtr<nsIInputStream> stringStream;
-  NS_NewCStringInputStream(getter_AddRefs(stringStream), str);
+  NS_NewByteInputStream(getter_AddRefs(stringStream), chrs, checkedLen.value(),
+                        NS_ASSIGNMENT_ADOPT);
 
   nsCOMPtr<nsIObjectInputStream> stream =
     NS_NewObjectInputStream(stringStream);