libdw: Handle FORM_GNU_strp_alt for files with string data only in dwz file.

dwarf_formstring should check that the dbg file from which we want the
strp data actually has a IDX_debug_str section.

Reported-By: Tom Tromey <tromey@redhat.com>
Signed-off-by: Mark Wielaard <mjw@redhat.com>
7 files changed