pua_xmpp: fix compilation warning

> xmpp2simple.c:372:10: warning: expression which evaluates to zero treated as a null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
>                 slash= '\0';
>                        ^~~~
> 1 warning generated.

(cherry picked from commit c3bbe6e)

modules/pua_xmpp/xmpp2simple.c

@@ -374,7 +374,7 @@ int build_publish(xmlNodePtr pres_node, int expires)
 			return -1;
 		}
 		strcpy(resource, slash+1);
-		slash= '\0';
+		slash= NULL;
 	}	
 	pres_uri= euri_xmpp_sip(uri);
 	xmlFree(uri);