pimd: Fix crash with debug and ifp changes

pimd: Fix crash with debug and ifp changes

Certain interface flapping events can cause a lookup
that does not find any ifp pointer.  This is only causing
a crash in the `debug pim zebra` command due to only needing
to lookup the interface for it's name.

Modify code to ensure we have a valid pointer.  Follow other
debug statements lead in the same function for what to display
when an interface does not currently exist.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>