[OpenMP] Fixed a bug when displaying affinity

Currently the affinity format string has initial value. When users set
the format via OMP_AFFINITY_FORMAT, it will overwrite the format string. However,
when copying the format, the tailing null is missing. As a result, if the user
format string is shorter than default value, the remaining part in the default
value still makes effort. This bug is not exposed because the test case doesn't
check the end of a string. It only checks whether given output "contains" the
check string.

Reviewed By: AndreyChurbanov

Differential Revision: https://reviews.llvm.org/D91309