USB: Make usb_buffer_free() NULL-safe

kfree() handles NULL arguments which is handy in error handling paths as one
does need to  insert bunch of ifs. How about making usb_buffer_free() do the
same?

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>