usb: gadget: f_fs: fix NULL pointer dereference when there are no strings

If the descriptors do not need any strings and user space sends empty
set of strings, the ffs->stringtabs field remains NULL.  Thus
*ffs->stringtabs in functionfs_bind leads to a NULL pointer
dereferenece.

The bug was introduced by commit [fd7c9a00: “use usb_string_ids_n()”].

While at it, remove double initialisation of lang local variable in
that function.

ffs->strings_count does not need to be checked in any way since in
the above scenario it will remain zero and usb_string_ids_n() is
a no-operation when colled with 0 argument.

Cc: <stable@vger.kernel.org>  # v2.6.36+
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_fs.c

@@ -1483,12 +1483,14 @@ static int functionfs_bind(struct ffs_data *ffs, struct usb_composite_dev *cdev)
 	ffs->ep0req->context = ffs;
 
 	lang = ffs->stringtabs;
-	for (lang = ffs->stringtabs; *lang; ++lang) {
+	if (lang) {
+		for (; *lang; ++lang) {
 			struct usb_string *str = (*lang)->strings;
 			int id = first_id;
 			for (; str->s; ++id, ++str)
 				str->id = id;
 		}
+	}
 
 	ffs->gadget = cdev->gadget;
 	ffs_data_get(ffs);