repofile: Avoid segfault if querying child in non-directory

The user might "ostree ls /usr/bin/bash/blah", which previously would
segfault.

A somewhat related future enhancement here would be for "ostree ls" to
follow symbolic links.

Reported-by: Dusty Mabe <dustymabe@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=733476

tests/test-basic.sh

@@ -267,6 +267,13 @@ cd ${test_tmpdir}
 $OSTREE ls test2
 echo "ok ls with no argument"
 
+cd ${test_tmpdir}
+if $OSTREE ls test2 /baz/cow/notadir 2>errmsg; then
+    assert_not_reached
+fi
+assert_file_has_content errmsg "Not a directory"
+echo "ok ls of not a directory"
+
 cd ${test_tmpdir}
 $OSTREE show test2
 echo "ok show with non-checksum"