libdwfl: Only intern CU when not EOF marker and cuoff points to a DIE.

libdwfl: Only intern CU when not EOF marker and cuoff points to a DIE.

We need to check the cuoff points to a real Dwarf_Die before trying to
intern the cu with tsearch. Otherwise bogus keys might end up in the
search tree with NULL cus. That will cause crashes in compare_cukey
during next insertion or deletion of cus. We also don't want to insert
the EOF marker and unconditionally tdestroy the lazy_cu_root. The EOF
could be caused by bad DWARF from a bogus agranges entry.

https://bugzilla.redhat.com/show_bug.cgi?id=1170810#c30

Signed-off-by: Mark Wielaard <mjw@redhat.com>