USB: diag: Add check for ctxt in usb_diag_request_size()

Diag char driver calls usb_diag_request_size() to get max request
size supported by underlying USB hardware. If there is race between
disable() and this API call, it results in accessing NULL pointer
dereference. Hence fix this by returning zero in case ctxt is NULL.

Change-Id: I51274db0af4cfbc8ba975744f55ad17ec311e32e
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>