Linux: Fix UTC offset setting in settimeofday for __TIMESIZE != 64

The time argument is NULL in this case, and attempt to convert it
leads to a null pointer dereference.

This fixes commit d2e3b697
("y2038: linux: Provide __settimeofday64 implementation").

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>