Bug 1323847: P3. Don't allocate data for empty buffer. r=gerald

MozReview-Commit-ID: I67RmFZpRDy

--HG--
extra : rebase_source : 19dcb4530dd053dc4b8735d872a4cb2e7adc2a6e

dom/media/MediaData.h

@@ -220,6 +220,10 @@ private:
   // Returns false if memory couldn't be allocated.
   bool EnsureCapacity(size_t aLength)
   {
+    if (!aLength) {
+      // No need to allocate a buffer yet.
+      return true;
+    }
     const CheckedInt<size_t> sizeNeeded =
       CheckedInt<size_t>(aLength) * sizeof(Type) + AlignmentPaddingSize();