[BOLT] Fix a bug related to iterators in ReorderData pass

If `Itr` is the last element and then `std::next(Itr)` will be
`Range.end()`, so that the statement `std::next(Itr)->second` is
a UB.

Reviewed By: yota9, maksfb

Differential Revision: https://reviews.llvm.org/D159177