usb: gadget: composite: Fix NULL pointer dereference

commit f563d230903210acc ("usb: gadget: composite: add req_match method
to usb_function") accesses cdev->config even before set config
is invoked causing a NULL pointer dereferencing error while running
Lecroy Mass Storage Compliance test.

Fix it here by accessing cdev->config only if it is non NULL.

Fixes: commit f563d230903210acc ("usb: gadget: composite: add req_match
method to usb_function").

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/composite.c

@@ -1758,10 +1758,13 @@ unknown:
 		 * take such requests too, if that's ever needed:  to work
 		 * in config 0, etc.
 		 */
-		list_for_each_entry(f, &cdev->config->functions, list)
-			if (f->req_match && f->req_match(f, ctrl))
-				goto try_fun_setup;
-		f = NULL;
+		if (cdev->config) {
+			list_for_each_entry(f, &cdev->config->functions, list)
+				if (f->req_match && f->req_match(f, ctrl))
+					goto try_fun_setup;
+			f = NULL;
+		}
+
 		switch (ctrl->bRequestType & USB_RECIP_MASK) {
 		case USB_RECIP_INTERFACE:
 			if (!cdev->config || intf >= MAX_CONFIG_INTERFACES)