trx_assign_rseg_low(): Simplify the debug check.

trx_rseg_t::reinit(): Reset the skip_allocation() flag.
This logic was broken in the merge
commit 3e2ad0e9
of commit 0de3be8c
(that is, innodb_undo_log_truncate=ON would never be "completed").

Tested by: Matthias Leich