snmp_output_err_resp, snmp_output_resp: allocate object using calloc, not
on the stack

Some of the callers try to determine whether or not object is valid by
testing the value for NULL, which will never be true if it's a stack value,
so in order to be clear and correct down the call stack, use a heap
allocated object.

This also addresses a Coverity issue by initializing all of object via
calloc

MFC after: 1 week
Reported by: Coverity
CID: 1006392