libdw: Fix crashing on illegal/zero Dwarf_Die.

libdw: Fix crashing on illegal/zero Dwarf_Die.

In some cases we create an illegal Dwarf_Die by clearing all fields.
The idea is that dwarf_tag () on such a Dwarf_Die will return
DW_TAG_invalid, to indicate that the Dwarf_Die is unusable (and other
functions will also return errors).  But when "reconstructing" the
Dwarf_Die addr we might use the cu before realizing the Dwarf_Die is
invalid.  Fix this with an explicit NULL check and add a testcase.

Signed-off-by: Mark Wielaard <mark@klomp.org>