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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44527

(cherry picked from commit b600644fdd6cefb1b90d76fdd5aa595946611a7d)