[arraybuffer] Set DataHandle length only if data is provided

When the DataHandle is created in ArrayBufferContents::CreateDataHandle,
the provided {data} allocated with AllocateMemoryOrNull can be nullptr.
If so, we don't want to store the length in the new DataHandle. To avoid
an inconsistent state.

R=haraken@chromium.org

Bug: chromium:1008840
Change-Id: Ia9bb106a82b5c2f884b8a4e09ebd4f6ca0be9152
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865220Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706857}

third_party/blink/renderer/platform/wtf/typed_arrays/array_buffer_contents.h

@@ -64,7 +64,7 @@ class WTF_EXPORT ArrayBufferContents {
                DataDeleter deleter,
                void* deleter_info)
         : data_(data),
-          data_length_(length),
+          data_length_(data ? length : 0),
           deleter_(deleter),
           deleter_info_(deleter_info) {}
     // Move constructor