upstream commit

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-ID: b7bcd2e82fc168a8eff94e41f5db336ed986fed0
