usb: gadget: composite: fix dereference after null check coverify warning

commit c526c62d upstream.

cdev->config is checked for null pointer at above code, so cdev->config
might be null, fix it by adding null pointer check.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>