x86/intr: Handle case of disabling MSI after release

[jhibbits]

Summary

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.

[kib]

sys/x86/x86/msi.c

222

Multi-line comment should have a blank line before it. It also should formatted like this:

/*
 * Interrupt sources are always registered, but never unregistered.
...

[jhibbits]

Address @kib's feedback.