libdwfl: Never relocate value against section zero (load address).

libdwfl: Never relocate value against section zero (load address).

This really is just a robustify patch in case section zero got the
wrong section flags set. In that case __libdwfl_relocate_value might
call dwfl_offline_section_address which might assert (because it isn't
prepared to handle section zero). elf_nextscn will never see section zero.
So be explicit in dwfl_offline_section_address and immediately assert
when shndx is zero. And handle section zero immediately by not relocating
the value in __libdwfl_relocate_value.

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