libnm-util: verify connection also for self-assignment in replace_settings_from_conne...

libnm-util: verify connection also for self-assignment in replace_settings_from_connection()

nm_connection_replace_settings_from_connection() would return whether the
connection verifies at the end of the operation. While that is not very
useful, the API is like that and cannot be changed.

For consistency, also perform the verification step in case of self-assignment.
Self-assigment is anyway a case that probably never happens.