drivers/usb/class/cdc-acm.c: clear dangling pointer

On some failures, the country_code field of an acm structure is freed
without freeing the acm structure itself.  Elsewhere, operations including
memcpy and kfree are performed on the country_code field.  The patch sets
the country_code field to NULL when it is freed, and likewise sets the
country_code_size field to 0.

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Oliver Neukum <oneukum@suse.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>