src/flash/nor/virtual: handle null pointers

src/flash/nor/virtual: handle null pointers

Either of protect and protect_check driver callbacks can be NULL. In
virtual_protect, reuse flash_driver_protect which checks for that case
and generates a nice error message and return code. In
virtual_protect_check, there is no corresponding flash_driver_*
function, so add the NULL check directly.

Change-Id: Ia63d85ede640a8382cf5cad0760f5d1ffe4d7cfe
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/5782
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>