[Clang][OpenMP] Bail out early if `Scope` is nullptr in case of any crash

When there is any compile error, clang still tries to compile as many code as
possible, therefore `Scope` can be `nullptr` here. However, we didn't check it
beforehand, causing compiler crash.

Fix #59944.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D142233