Fix edge case in stack trace formatting.

Bug description: in strict mode, null as receiver is not implicitly converted
to the global object, so that when formatting the stack trace, the receiver of
the stack frame is null. The IS_OBJECT check returns true for null, but
%GetDataProperty expected a JSObject, which results in a failed RUNTIME_ASSERT.

R=mvstanton@chromium.org
BUG=237617

Review URL: https://chromiumcodereview.appspot.com/15670003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14797 ce2b1a6d-e550-0410-aec6-3dcde31c8c00