x86/IRQ: avoid double unlock in map_domain_pirq()

x86/IRQ: avoid double unlock in map_domain_pirq()

Forever since its introduction the main loop in the function dealing
with multi-vector MSI had error exit points ("break") with different
properties: In one case no IRQ descriptor lock is being held.
Nevertheless the subsequent error cleanup path assumed such a lock would
uniformly need releasing. Identify the case by setting "desc" to NULL,
thus allowing the unlock to be skipped as necessary.

This is CVE-2024-31143 / XSA-458.

Coverity ID: 1605298
Fixes: d1b6d0a02489 ("x86: enable multi-vector MSI")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>