Fix file-name-case-insensitive-p in ffap (bug#56443)

Don't crash if the file name argument to file-name-case-insensitive-p,
after expansion, doesn't have a parent directory.  This occurs
when calling ffap on something that looks like an email address.

* src/fileio.c (Ffile_name_case_insensitive_p): Return nil if no file
or parent directory could be found.
* test/src/fileio-tests.el (fileio-tests--identity-expand-handler)
(fileio--file-name-case-insensitive-p): New test.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/fileio.c b/src/fileio.c
index d07e62a1212..9697f6c8cf1 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2601,9 +2601,9 @@ is case-insensitive.  */)
       if (err <= 0)
         return err < 0 ? Qt : Qnil;
       Lisp_Object parent = file_name_directory (filename);
-      /* Avoid infinite loop if the root has trouble
-         (impossible?).  */
-      if (!NILP (Fstring_equal (parent, filename)))
+      /* Avoid infinite loop if the root has trouble (if that's even possible).
+         Without a parent, we just don't know and return nil as well.  */
+      if (!STRINGP (parent) || !NILP (Fstring_equal (parent, filename)))
         return Qnil;
       filename = parent;
     }