MFstable/11 r310877:

MFstable/11 r310877:

MFC r310455:

Clarify failure in snmp_output(..) with call to snmp_pdu_decode

- Explicitly test snmp_pdu_encode against SNMP_CODE_OK instead of assuming
  any non-zero value is bad.
- Print out the code before calling abort() to give the end-user something
  actionable to debug without having to recompile the binary, since the
  core might not have these details.

git-svn-id: svn://svn.freebsd.org/base/stable/10@310878 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f