lib: Don't delete uninitialized pointers

In the error-handling paths of notmuch_database_open, we call
notmuch_database_close, which "delete"s several objects referenced by
the notmuch_database_t object.  However, some of these pointers may be
uninitialized, resulting in undefined behavior.  Hence, allocate the
notmuch_database_t with talloc_zero to make sure these pointers are
NULL so that "delete"ing them is harmless.