Check for NULL pointer in partitionAllocGeneric fast path.

(Reland of a small piece of
http://src.chromium.org/viewvc/blink?revision=164447&view=revision
which was reverted.)

BUG=331699
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/124963002

git-svn-id: svn://svn.chromium.org/blink/trunk@164502 bbb929c8-8fbe-4397-9dbb-9b2b20218538