[OpenMP][NFC] libomp: fix wrong debug assertion.

Normalized bounds of chunk of iterations to steal from are inclusive,
so upper bound should not be decremented in expression to check.
Problem was in attempt to steal iterations 0:0, that caused assertion after
wrong decrement. Reported in comment to https://reviews.llvm.org/D103648.

Differential Revision: https://reviews.llvm.org/D104880