ntlm_wb: Fix memory leaks in ntlm_wb_response

When erroring out on a request being too large, the existing buffer was
leaked. Fix by explicitly freeing on the way out.

Closes #2966
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

lib/curl_ntlm_wb.c

@@ -295,6 +295,7 @@ static CURLcode ntlm_wb_response(struct connectdata *conn,
 
     if(len_out > MAX_NTLM_WB_RESPONSE) {
       failf(conn->data, "too large ntlm_wb response!");
+      free(buf);
       return CURLE_OUT_OF_MEMORY;
     }