this is similar to the fix that I did recently for rerequesting
dwarf_tag for DIE with zero abbrev code, except this time the DIE has
non-zero invalid code. On such a DIE, the following SEGVs libdw:
die = dwarf_offdie (dw, 11, &die_mem);
die = dwarf_offdie (dw, 11, &die_mem)
The fix and a test case are on pmachata/rerequest-tag.