The change in glibc commit d58ab810a6e325cc351684d174c48cabce01bcc1
(author in CC):

From commit description:"[...] Also avoid an unnecessary call to
strcspn after the last token by adding an early exit for an empty
string.[...]"

Important code change:
/* Parse S into tokens separated by characters in DELIM.
@@ -45,11 +41,17 @@
 char *
 __strtok_r (char *s, const char *delim, char **save_ptr)
 {
-  char *token;
+  char *end;

   if (s == NULL)
     s = *save_ptr;

+  if (*s == '\0')
+    {
+      *save_ptr = s;
+      return NULL;
+    }
+

may result in the mentioned segmentation fault if the char *str passed
to strtok_r is a NULL (for 1st call).

Checked glibc versions:
~/git-repos/glibc:release/2.25/master$ git tag
--contain=d58ab810a6e325cc351684d174c48cabce01bcc1
changelog-ends-here
glibc-2.25
glibc-2.25.90
glibc-2.26
glibc-2.26.9000
glibc-2.27
glibc-2.27.9000
glibc-2.28
glibc-2.28.9000
glibc-2.29
glibc-2.29.9000
glibc-2.30
glibc-2.30.9000
glibc-2.31
glibc-2.31.9000

cheers,
Marcin