ValueSerializer: Check for zero length before casting to FixedDoubleArray.

Even though the elements kind is FAST_DOUBLE_ELEMENTS, if length is zero
the isolate's empty_fixed_array is used. It's illegal to cast this to
FixedDoubleArray, so we avoid the cast.

BUG=chromium:686479

Review-Url: https://codereview.chromium.org/2665313003
Cr-Commit-Position: refs/heads/master@{#42867}

src/value-serializer.cc

@@ -554,6 +554,9 @@ Maybe<bool> ValueSerializer::WriteJSArray(Handle<JSArray> array) {
         break;
       }
       case FAST_DOUBLE_ELEMENTS: {
+        // Elements are empty_fixed_array, not a FixedDoubleArray, if the array
+        // is empty. No elements to encode in this case anyhow.
+        if (length == 0) break;
         Handle<FixedDoubleArray> elements(
             FixedDoubleArray::cast(array->elements()), isolate_);
         for (; i < length; i++) {