ntlm_wb: Fix theoretical memory leak

Static analysis indicated that my commit 9008f3d564 ("ntlm_wb: Fix
hard-coded limit on NTLM auth packet size") introduced a potential
memory leak on an error path, because we forget to free the buffer
before returning an error.

Fix this.

Although actually, it never happens in practice because we never *get*
here with state == NTLMSTATE_TYPE1. The state is always zero. That
might want cleaning up in a separate patch.

Reported-by: Terri Oda

lib/curl_ntlm_wb.c

@@ -306,7 +306,7 @@ static CURLcode ntlm_wb_response(struct connectdata *conn,
   if(state == NTLMSTATE_TYPE1 &&
      len_out == 3 &&
      buf[0] == 'P' && buf[1] == 'W')
-    return CURLE_REMOTE_ACCESS_DENIED;
+    goto done;
   /* invalid response */
   if(len_out < 4)
     goto done;