ofagent: Fix a possible crash in arp responder

Be careful for exceptions when feeding packet-in data,
which is generated by tenant VMs and thus can not be trusted,
to Ryu packet library.

Closes-Bug: #1365255
Change-Id: Ia8bacfb55def563a1b23a47709ae72bd4fce0fce
Merge "ofagent: Fix a possible crash in arp responder"