Fix gwbuf_rtrim

- If everything in the first buffer of a buffer chain is consumed,
  then the whole chain and not just the first buffer was freed.

NOTE: gwbuf_rtrim needs to be fixed so that it removes data from the
      tail of a chain and *not* from the end of the first buffer in
      a chain. That cannot ever be what is wanted.

server/core/buffer.c

@@ -640,7 +640,7 @@ gwbuf_rtrim(GWBUF *head, unsigned int n_bytes)
     if (GWBUF_EMPTY(head))
     {
         rval = head->next;
-        gwbuf_free(head);
+        gwbuf_free_one(head);
     }
     return rval;
 }