pcmcia: rsrc_nonstatic: Fix a NULL pointer dereference in __nonstatic_find_io_region()

In __nonstatic_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.

This bug was found by a static analyzer. The analysis employs
differential checking to identify inconsistent security operations
(e.g., checks or kfrees) between two code paths and confirms that the
inconsistent operations are not recovered in the current function or
the callers, so they constitute bugs.

Note that, as a bug found by static analysis, it can be a false
positive or hard to trigger. Multiple researchers have cross-reviewed
the bug.

Builds with CONFIG_PCCARD_NONSTATIC=y show no new warnings,
and our static analyzer no longer warns about this code.

Fixes: 49b1153adf ("pcmcia: move all pcmcia_resource_ops providers into one module")
Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
[linux@dominikbrodowski.net: Fix typo in commit message]
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

drivers/pcmcia/rsrc_nonstatic.c

@@ -690,6 +690,9 @@ static struct resource *__nonstatic_find_io_region(struct pcmcia_socket *s,
 	unsigned long min = base;
 	int ret;
 
+	if (!res)
+		return NULL;
+
 	data.mask = align - 1;
 	data.offset = base & data.mask;
 	data.map = &s_data->io_db;