MFC r310957,r310958,r310960:

r310957:

Use strlcpy when copying com to pdu->community to avoid potential
buffer overruns

CID: 1006823, 1006824

r310958:

Initialize ret to SNMPD_INPUT_OK at the top of snmp_input_start(..) to
avoid returning an uninitialized value

There are some really complicated, snakey if-statements combined with
switch statements that could result in an invalid value being returned
as ret

CID: 1006551

r310960:

Similar to r310954, set .len to 0 on malloc failure and to len only
on success