netdump: Check the return value of ifunit_ref()

We may fail to match if the specific interface doesn't exist or was
renamed.

Approved by: re (gjb)
PR: 273715
Reported by: grembo
MFC after: 1 week

(cherry picked from commit d94d07d58141dcff48f01c6b3e5a31de9d7a7938)
(cherry picked from commit d08ef6f7da3943dc49999988e6c2ac03644bdaf1)