usb: gadget: composite: Fix possible double free memory bug

composite_dev_cleanup call from the failure of configfs_composite_bind
frees up the cdev->os_desc_req and cdev->req. If the previous calls of
bind and unbind is successful these will carry stale values.

Consider the below sequence of function calls:
configfs_composite_bind()
        composite_dev_prepare()
                - Allocate cdev->req, cdev->req->buf
        composite_os_desc_req_prepare()
                - Allocate cdev->os_desc_req, cdev->os_desc_req->buf
configfs_composite_unbind()
        composite_dev_cleanup()
                - free the cdev->os_desc_req->buf and cdev->req->buf
Next composition switch
configfs_composite_bind()
        - If it fails goto err_comp_cleanup will call the
	  composite_dev_cleanup() function
        composite_dev_cleanup()
	        - calls kfree up with the stale values of cdev->req->buf and
		  cdev->os_desc_req from the previous configfs_composite_bind
		  call. The free call on these stale values leads to double free.

Hence, Fix this issue by setting request and buffer pointer to NULL after
kfree.

Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>