dummynet: check for ifp on all PROTO_LAYER2 packets

When we extended the switch statement to allow for PROTO_LAYER2 |
PROTO_IPV6 in c21cbaca2b we didn't extend the check for a non-NULL
struct ifnet pointer.
Happily the only PROTO_IPV6 case is pf's layer 2 support, which always
provides one.

Reported by: Coverity (CID 1490459)
Sponsored by: Rubicon Communications, LLC ("Netgate")