Set u.ext_data to null after it has been freed

SEGV_MAPPER crash could happen in MetaData::typed_data::freeStorage.
The faulty address could be both random pointers and deadbaad. It
seems like free could be called more than once on u.ext_data. Set
u.ext_data to null after it has been freed as there already is a
null check before trying to free it.

Change-Id: I710c49f9c231bd8852159914c0b2add58a685f0e