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>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I4c09a97dd293de404f2e8782101a7b0d1d6e97c9