usb: gadget: eliminate NULL pointer dereference (bugfix)

usb: gadget: eliminate NULL pointer dereference (bugfix)

commit 92b0abf80c5c5f0e0d71d1309688a330fd74731b upstream.

usb: gadget: eliminate NULL pointer dereference (bugfix)

This patch fixes a bug which causes NULL pointer dereference in
ffs_ep0_ioctl. The bug happens when the FunctionFS is not bound (either
has not been bound yet or has been bound and then unbound) and can be
reproduced with running the following commands:

$ insmod g_ffs.ko
$ mount -t functionfs func /dev/usbgadget
$ ./null

where null.c is:

#include <fcntl.h>
#include <linux/usb/functionfs.h>

int main(void)
{
	int fd = open("/dev/usbgadget/ep0", O_RDWR);
	ioctl(fd, FUNCTIONFS_CLEAR_HALT);

	return 0;
}

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>