Completed cosing of substring.

ZString.cpp

@@ -171,6 +171,13 @@ namespace coreutils {
    }
    
    ZString ZString::substring(int start, int len) {
+      char *end = data + length;
+      char *startChar = data + start;
+      char *endChar = startChar + len;
+      char newlen = endChar > end ? endChar - end: len;
+      if(startChar < end) {
+	 return ZString(startChar, newlen);
+      }
       return ZString();
    }