Fixed a segfault during text concatenation when validating a node tree:

* xmlschemas.c xmlstring.c: Fixed a segfault during
  text concatenation when validating a node tree:
  xmlStrncat was called with a @len of -1; but unlike
  xmlStrncatNew, it does not calculate the length
  automatically in such a case (reported by Judy Hay
  on the mailing list).
  Updated the descriptions of the involved string
  functions to note this.
diff --git a/xmlschemas.c b/xmlschemas.c
index f7f7567..9518999 100644
--- a/xmlschemas.c
+++ b/xmlschemas.c
@@ -26336,7 +26336,9 @@
 	    default:
 		break;
 	}
-    } else {	
+    } else {
+	if (len < 0)
+	    len = xmlStrlen(value);
 	/*
 	* Concat the value.
 	*/