added check for external reference in xmlRelaxNGGetElements (bug 137718)

* relaxng.c: added check for external reference in
  xmlRelaxNGGetElements (bug 137718)
* test/relaxng/rngbug-001.*, result/relaxng/rngbug-001*: added
  regression test for above
diff --git a/relaxng.c b/relaxng.c
index 5c01dcf..cbc9073 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -3978,7 +3978,8 @@
                    (cur->type == XML_RELAXNG_OPTIONAL) ||
                    (cur->type == XML_RELAXNG_PARENTREF) ||
                    (cur->type == XML_RELAXNG_REF) ||
-                   (cur->type == XML_RELAXNG_DEF)) {
+                   (cur->type == XML_RELAXNG_DEF) ||
+		   (cur->type == XML_RELAXNG_EXTERNALREF)) {
             /*
              * Don't go within elements or attributes or string values.
              * Just gather the element top list