ln: with -sr, don't segfault for a TARGET of ''

ln: with -sr, don't segfault for a TARGET of ''

Prior to this change, "ln -sr '' F" would segfault, attempting
to read path2[1] in relpath.c's path_common_prefix function.
This problem arises whenever canonicalize_filename_mode returns
NULL.
* src/ln.c (convert_abs_rel): Call relpath only when
both canonicalize_filename_mode calls return non-NULL.
* tests/ln/relative.sh: Add a test to trigger this failure.
* THANKS.in: List reporter's name/address.
* NEWS (Bug fixes): Mention it.
Reported by Erik Bernstein in 739752@bugs.debian.org.
Fixes http://bugs.gnu.org/17010.