PCI/MSI: Check for NULL affinity mask in pci_irq_get_affinity()

If msi_setup_entry() fails to allocate an affinity mask, it logs a message
but continues on and allocates an MSI entry with entry->affinity == NULL.

Check for this case in pci_irq_get_affinity() so we don't try to
dereference a NULL pointer.

[bhelgaas: changelog]
Fixes: ee8d41e5 "pci/msi: Retrieve affinity for a vector"
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
CC: Thomas Gleixner <tglx@linutronix.de>

drivers/pci/msi.c

@@ -1292,7 +1292,8 @@ const struct cpumask *pci_irq_get_affinity(struct pci_dev *dev, int nr)
 	} else if (dev->msi_enabled) {
 		struct msi_desc *entry = first_pci_msi_entry(dev);
 
-		if (WARN_ON_ONCE(!entry || nr >= entry->nvec_used))
+		if (WARN_ON_ONCE(!entry || !entry->affinity ||
+				 nr >= entry->nvec_used))
 			return NULL;
 
 		return &entry->affinity[nr];