Fix mandoc_normdate() and the way it is used.

Fix mandoc_normdate() and the way it is used.
In the past, it could return NULL but the calling code wasn't prepared
to handle that.  Make sure it always returns an allocated string.
While here, simplify the code by handling the "quick" attribute
inside mandoc_normdate() rather than at multiple callsites.

Triggered by deraadt@ pointing out
that snprintf(3) error handling was incomplete in time2a().