lib: pair VA_COPY with va_end.

A va_copy creates a initialised va_list, as if a va_start had been done on it.
Therefore, pedantically, a va_end should also be done on it. On most platforms
this is a no-op, and for those where it isn't, the pairing is important.

This change matches parallel change in Dovecot. Also signalled by Coverity.