tcp hpts: improve consistency

The target_slot argument of max_slots_available() can be NULL.
Therefore, check for this in all places.
Right now, all callers provide non-NULL pointer.

Reported by: Coverity Scan
CID: 1527732
Reviewed by: rrs
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44527