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: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
CC: Thomas Gleixner <tglx@linutronix.de>