lib/create: fix memory leak, ensure *database=NULL on error

This code previously relied on _finish_open to free the notmuch struct
on errors (except for the case of database == NULL, which was a
potential double free). When we removed those frees from _finish_open,
we introduced a (small) memory leak.

In this commit, fix the memory leak, and harmonize the on-error
behaviour with n_d_open_with_config.