Do not return out of bound pointers from intr_lookup_source().

This hardens the code against driver and upper level bugs causing
invalid indexes used, e.g. on msi release.

Reported by:	gallatin
Reviewed by:	gallatin, hselasky
Sponsored by:	Mellanox Technologies
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D14470

sys/x86/x86/intr_machdep.c

@@ -178,6 +178,8 @@ struct intsrc *
 intr_lookup_source(int vector)
 {
 
+	if (vector < 0 || vector >= nitems(interrupt_sources))
+		return (NULL);
 	return (interrupt_sources[vector]);
 }