Use calloc instead of malloc with buffers in snmp_{recv,send}_packet

This doesn't fix the issue noted in the PR, but at the very least it
cleans up the error so it looks a bit more sane, and in the event
that bsnmp did wander off into the weeds, the likelihood of it
crashing with more sensible output is greater, in my opinion

MFC counter set high so I have enough time to resolve the real
underlying bug in bsnmpwalk

MFC after: 1 month
PR: 215721