backends: Check type DIE exists before calling dwarf_tag ().

backends: Check type DIE exists before calling dwarf_tag ().

dwarf_attr () or dwarf_form () functions leave typedie NULL when they fail
because of missing attribute or unexpected form. In such cases first check
the DIE exists and return error instead of calling dwarf_tag () and crashing.

This also happens in the testsuite with native tests when elfutils is build
without DWZ support on a distro that uses DWZ DWARF compression on system
libraries. Only the backends used dwarf_tag () directly without checking,
all other uses in elfutils already checked whether the given DIE was NULL.

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