AMD IOMMU: only translate remapped IO-APIC RTEs

1aeb1156fa ("x86 don't change affinity with interrupt unmasked")
introducing RTE reads prior to the respective interrupt having got
enabled for the first time uncovered a bug in 2ca9fbd739 ("AMD IOMMU:
allocate IRTE entries instead of using a static mapping"): We obviously
shouldn't be translating RTEs for which remapping didn't get set up
yet.

Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/amd/iommu_intr.c
+++ b/xen/drivers/passthrough/amd/iommu_intr.c
@@ -365,15 +365,17 @@ unsigned int amd_iommu_read_ioapic_from_
     unsigned int apic, unsigned int reg)
 {
     unsigned int val = __io_apic_read(apic, reg);
+    unsigned int pin = (reg - 0x10) / 2;
+    unsigned int offset = ioapic_sbdf[IO_APIC_ID(apic)].pin_2_idx[pin];
 
-    if ( !(reg & 1) )
+    if ( !(reg & 1) && offset < INTREMAP_ENTRIES )
     {
-        unsigned int offset = val & (INTREMAP_ENTRIES - 1);
         u16 bdf = ioapic_sbdf[IO_APIC_ID(apic)].bdf;
         u16 seg = ioapic_sbdf[IO_APIC_ID(apic)].seg;
         u16 req_id = get_intremap_requestor_id(seg, bdf);
         const u32 *entry = get_intremap_entry(seg, req_id, offset);
 
+        ASSERT(offset == (val & (INTREMAP_ENTRIES - 1)));
         val &= ~(INTREMAP_ENTRIES - 1);
         val |= get_field_from_reg_u32(*entry,
                                       INT_REMAP_ENTRY_INTTYPE_MASK,