tree: Ignore NULL address pointer for phy slot lookup

The PCI physical slot lookup works obviously only for physical cards.
Thus do not try to dereference the address pointer if it is a NULL
pointer.

Fixes: 42ac453 ("tree: Add PCI physical slot number for controller")
Signed-off-by: Daniel Wagner <[email protected]>

Author: dwsuse

src/nvme/tree.c

@@ -1274,6 +1274,9 @@ static char *nvme_ctrl_lookup_phy_slot(nvme_root_t r, const char *address)
 	DIR *slots_dir;
 	struct dirent *entry;
 
+	if (!address)
+		return NULL;
+
 	slots_dir = opendir(nvme_slots_sysfs_dir);
 	if (!slots_dir) {
 		nvme_msg(r, LOG_WARNING, "failed to open slots dir %s\n",