glx: Fix mask truncation in __glXGetAnswerBuffer [CVE-2014-8093 6/6]

On a system where sizeof(unsigned) != sizeof(intptr_t), the unary
bitwise not operation will result in a mask that clears all high bits
from temp_buf in the expression:
        temp_buf = (temp_buf + mask) & ~mask;

Signed-off-by: Robert Morell <rmorell@nvidia.com>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>