Fix userprog tests for clang

Recent clang version, detect attemps to dereference or call NULL, and
eliminate the code.

../../tests/userprog/bad-read.c:11:9: warning: indirection of
non-volatile null pointer will be deleted, not trap [-Wnull-dereference]
        *(int *) NULL);
        ^~~~~~~~~~~~~

This patchs adds the volatile modifier as necessary.
It allows to use clang to compile the tests.