[API] Check for NULL external data pointer in ArrayBuffer::New()

[API] Check for NULL external data pointer in ArrayBuffer::New()

Embedders must not provide invalid pointers for external backing stores.

BUG=chromium:522128
LOG=n
R=cbruni@chromium.org

Review URL: https://codereview.chromium.org/1289373004

Cr-Commit-Position: refs/heads/master@{#30243}