scripts: elf_helper.py: fix C++ template constexpr value processing

Some DWARF symbols for members of template classes members such as
numeric_limits<unsigned int> reference are static constexpr values
that do not have a data member location.  Avoid attempting to
dereference the value for that attribute when it isn't present.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>