fixed validation of maxLength with no content using patch submitted by

* xmlschemastypes.c: fixed validation of maxLength with no
  content using patch submitted by Eric Haszlakiewicz
  (bug 133259)
diff --git a/xmlschemastypes.c b/xmlschemastypes.c
index 6ffba47..b48a549 100644
--- a/xmlschemastypes.c
+++ b/xmlschemastypes.c
@@ -3528,7 +3528,8 @@
 		    	len = xmlSchemaNormLen(value);
 		    	break;
 		    case XML_SCHEMAS_STRING:
-		    	len = xmlUTF8Strlen(value);
+		        if (value != NULL)
+			    len = xmlUTF8Strlen(value);
 		    	break;
 		    default:
 		        TODO