gicv2: Don't translate irqInvalid into array index

This also adds a check that translating a core + irq doesn't result in
an index that is irqInvalid.

include/arch/arm/arch/machine/gic_v2.h

@@ -207,12 +207,12 @@ static inline interrupt_t getActiveIRQ(void)
     }
 
     if (IS_IRQ_VALID(active_irq[CURRENT_CPU_INDEX()])) {
-        irq = active_irq[CURRENT_CPU_INDEX()] & IRQ_MASK;
+        irq = CORE_IRQ_TO_IDX(CURRENT_CPU_INDEX(), active_irq[CURRENT_CPU_INDEX()] & IRQ_MASK);
     } else {
         irq = irqInvalid;
     }
 
-    return CORE_IRQ_TO_IDX(CURRENT_CPU_INDEX(), irq);
+    return irq;
 }
 
 /*

src/arch/arm/machine/gic_v2.c

@@ -168,6 +168,8 @@ void setIRQTrigger(irq_t irq, bool_t trigger)
 
 BOOT_CODE void initIRQController(void)
 {
+    /* irqInvalid cannot correspond to a valid IRQ index into the irq state array */
+    assert(INT_STATE_ARRAY_SIZE < irqInvalid);
     dist_init();
 }