A static analyzer complained about a couple instances of checking a
variable against NULL after already having dereferenced it.
- dmar_gas_alloc_region: remove the tautological NULL checks
- dmar_release_resources / dmar_fini_fault_log: don't deref unit->regs
unless initialized.
And while here, fix an inverted initialization check in dmar_fini_qi.
Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential revision: https://reviews.freebsd.org/D20263
244081120e