patch from Richard Jinks for XPath substring() function new set of tests

* xpath.c: patch from Richard Jinks for XPath substring() function
* result/XPath/expr/strings test/XPath/expr/strings: new set of tests
Daniel
diff --git a/xmlIO.c b/xmlIO.c
index 949d9c6..a540c06 100644
--- a/xmlIO.c
+++ b/xmlIO.c
@@ -143,6 +143,7 @@
         xmlInputCallbackTable[i].readcallback = NULL;
         xmlInputCallbackTable[i].closecallback = NULL;
     }
+    xmlInputCallbackInitialized = 0;
 
     xmlInputCallbackNr = 0;
     xmlInputCallbackInitialized = 0;
@@ -168,6 +169,7 @@
         xmlOutputCallbackTable[i].writecallback = NULL;
         xmlOutputCallbackTable[i].closecallback = NULL;
     }
+    xmlOutputCallbackInitialized = 0;
 
     xmlOutputCallbackNr = 0;
     xmlOutputCallbackInitialized = 0;
@@ -2116,7 +2118,7 @@
     int res = 0;
     int nbchars = 0;
     int buffree;
-    int needSize;
+    unsigned int needSize;
 
     if ((len <= MINLEN) && (len != 4)) 
         len = MINLEN;