[flang] Fix crash from bug#63099 (gfortran/regression/oldstyle_3.f90)

In InitElement() in flang/lib/Semantics/data-to-inits.cpp, don't
assume that values_.Location() can be called until after it has been
verified that values_.IsAtEnd() is not true.

Fixes https://github.com/llvm/llvm-project/issues/63099.

Differential Revision: https://reviews.llvm.org/D152183