pf: improve ASCONF chunk validation

When processing an ASCONF chunk we failed to verify that the chunk
length was at least 8 bytes. As a result we might end up passing a
negative length to pf_multihome_scan(). Fortunately this merely meant
the function did nothing, but we should discard such invalid packets, so
explicitly check for this.

MFC after: 1 week
Reported by: Mark Johnston
Sponsored by: Orange Business Services

(cherry picked from commit bea1c2fcd7839fd90a8ce96d6dc6a033779bc3c2)