x86/intr: Handle case of disabling MSI after release

Once an interrupt source is registered it's never deregistered.
However, when an MSI is released the pointer for it becomes NULLed out,
resulting in a NULL pointer dereference when attempting to disable the
now-released MSI source. Add NULL check to avoid this.

Reviewed by: kib
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D51624