pcmcia: Fix a NULL pointer dereference in __iodyn_find_io_region()

In __iodyn_find_io_region(), pcmcia_make_resource() is assigned to
res and used in pci_bus_alloc_resource(). There is a dereference of res
in pci_bus_alloc_resource(), which could lead to a NULL pointer
dereference on failure of pcmcia_make_resource().

Fix this bug by adding a check of res.

Cc: stable@vger.kernel.org
Fixes: 49b1153a ("pcmcia: move all pcmcia_resource_ops providers into one module")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>