Curl_timeleft: avoid returning "no timeout" by mistake

As this function uses return code 0 to mean that there is no timeout, it
needs to check that it doesn't return a time left value that is exactly
zero. It could lead to libcurl doing an extra 1000 ms select() call and
thus not timing out as accurately as it should.

I fell over this bug when working on the bug 3061535 but this fix does
not correct that problem alone, although this is a problem that needs to
be fixed.

Reported by: Rodric Glaser
Bug: http://curl.haxx.se/bug/view.cgi?id=3061535

lib/connect.c

@@ -174,6 +174,9 @@ long Curl_timeleft(struct connectdata *conn,
 
   /* substract elapsed time */
   timeout_ms -= Curl_tvdiff(*nowp, data->progress.t_startsingle);
+  if(!timeout_ms)
+    /* avoid returning 0 as that means no timeout! */
+    return -1;
 
   return timeout_ms;
 }