Port upstream commit, to be removed with v8.23:

  http://git.sv.gnu.org/cgit/coreutils.git/commit/?id=0093ac8d57

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

  ln -sr '' F no longer segfaults.  Now works as expected.
  [bug introduced with the --relative feature in coreutils-8.16]

The changes in NEWS and THANKS.in in the original patch have been omitted.

-----------------------------------------------
commit 0093ac8d57a0f1a16fd09d98f6a524dddb6053e7
Author: Jim Meyering <meyering@fb.com>
Date:   Thu Mar 13 17:05:04 2014 -0700

    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.
    Reported by Erik Bernstein in 739752@bugs.debian.org.
    Fixes http://bugs.gnu.org/17010.

---
 src/ln.c             |   16 ++++++++++------
 tests/ln/relative.sh |    5 +++++
 2 files changed, 15 insertions(+), 6 deletions(-)

Index: src/ln.c
===================================================================
--- src/ln.c.orig
+++ src/ln.c
@@ -139,13 +139,17 @@ convert_abs_rel (const char *from, const
   char *realdest = canonicalize_filename_mode (targetdir, CAN_MISSING);
   char *realfrom = canonicalize_filename_mode (from, CAN_MISSING);
 
-  /* Write to a PATH_MAX buffer.  */
-  char *relative_from = xmalloc (PATH_MAX);
-
-  if (!relpath (realfrom, realdest, relative_from, PATH_MAX))
+  char *relative_from = NULL;
+  if (realdest && realfrom)
     {
-      free (relative_from);
-      relative_from = NULL;
+      /* Write to a PATH_MAX buffer.  */
+      relative_from = xmalloc (PATH_MAX);
+
+      if (!relpath (realfrom, realdest, relative_from, PATH_MAX))
+        {
+          free (relative_from);
+          relative_from = NULL;
+        }
     }
 
   free (targetdir);
Index: tests/ln/relative.sh
===================================================================
--- tests/ln/relative.sh.orig
+++ tests/ln/relative.sh
@@ -45,4 +45,9 @@ mkdir web
 ln -sr latest web/latest
 test $(readlink web/latest) = '../release2' || fail=1
 
+# Expect this to fail with exit status 1, or to succeed quietly (freebsd).
+# Prior to coreutils-8.23, it would segfault.
+ln -sr '' F
+case $? in [01]) ;; *) fail=1;; esac
+
 Exit $fail