AX:Null pointer may be dereferenced in AXObjectCache.cpp

Added a check if newObj is not null and an assert in order to avoid dereferecing null pointer.
This is a patch from webkit.org: http://trac.webkit.org/changeset/154767

BUG=326935

Review URL: https://codereview.chromium.org/93543003

git-svn-id: svn://svn.chromium.org/blink/trunk@164523 bbb929c8-8fbe-4397-9dbb-9b2b20218538