xtensa: fix error paths in __get_user_{check,size}

Error paths in __get_user_check and __get_user_size directly assing 0 to
the result. It causes the following sparse warnings:

  sparse: warning: Using plain integer as NULL pointer

Convert 0 to the type pointed to by the user pointer before assigning it.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>