MFC r310957,r310958,r310960:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@311597 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f