Fix an unimplemented part in RNG value validation

Forgot to implement <optional> this was raised again
in https://bugzilla.redhat.com/show_bug.cgi?id=752393
as this make libxml2 fail to validate against ODF RNGs

Daniel

relaxng.c

@@ -8884,6 +8884,11 @@ xmlRelaxNGValidateValue(xmlRelaxNGValidCtxtPtr ctxt,
         case XML_RELAXNG_ZEROORMORE:{
                 xmlChar *cur, *temp;
 
+                if ((ctxt->state->value == NULL) ||
+                    (*ctxt->state->value == 0)) {
+                    ret = 0;
+                    break;
+                }
                 oldflags = ctxt->flags;
                 ctxt->flags |= FLAGS_IGNORABLE;
                 cur = ctxt->state->value;
@@ -8905,6 +8910,30 @@ xmlRelaxNGValidateValue(xmlRelaxNGValidCtxtPtr ctxt,
 		    xmlRelaxNGPopErrors(ctxt, 0);
                 break;
             }
+        case XML_RELAXNG_OPTIONAL:{
+                xmlChar *temp;
+
+                if ((ctxt->state->value == NULL) ||
+                    (*ctxt->state->value == 0)) {
+                    ret = 0;
+                    break;
+                }
+                oldflags = ctxt->flags;
+                ctxt->flags |= FLAGS_IGNORABLE;
+                temp = ctxt->state->value;
+                ret = xmlRelaxNGValidateValue(ctxt, define->content);
+                ctxt->flags = oldflags;
+                if (ret != 0) {
+                    ctxt->state->value = temp;
+                    if (ctxt->errNr > 0)
+                        xmlRelaxNGPopErrors(ctxt, 0);
+                    ret = 0;
+                    break;
+                }
+		if (ctxt->errNr > 0)
+		    xmlRelaxNGPopErrors(ctxt, 0);
+                break;
+            }
         case XML_RELAXNG_EXCEPT:{
                 xmlRelaxNGDefinePtr list;