Fix two rare edge cases: 1. If vasprintf() returns < 0,
do not access a NULL pointer in snmprintf(), and do not free() the pointer
returned from vasprintf() because on some systems other than OpenBSD, it
might be a bogus pointer. 2. If vasprintf() returns == 0, return 0 and ""
rather than -1 and NULL.
Besides, free(dst) is pointless after failure (not a bug).
One half OK martijn@, the other half OK deraadt@;
committing quickly before people get hurt.
Upstream-Regress-ID: b164f20923812c9bac69856dbc1385eb1522cba4