Fix crash in Fortran code

Fix crash in Fortran code

PR fortran/28801 points out a gdb crash that can be provoked by
certain Fortran code.  The bug is that f77_get_upperbound assumes the
property is either a constant or undefined, but in this case it is
PROP_LOCEXPR.

This patch fixes the crash by making this function (and the
lower-bound one as well) do the correct check before calling
'const_val'.

Thanks to Andrew for writing the test case.

Co-authored-by: Andrew Burgess <aburgess@redhat.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28801