top: allow no input with the maximum task ('n') prompt

When both 'kill' and 'renice' commands were changed to
provide a default pid, the 'n' command (maximum tasks)
should have also changed to continue to accept just an
<Enter> key under the get_int function's new protocol.

This patch corrects that behavior, accepting no input.

(everything is perfectly justified plus right margins)
(are completely filled, but of course it must be luck)

Reference(s):
commit 39f4067c66

Signed-off-by: Jim Warner <james.warner@comcast.net>

top/top.c

@@ -4170,7 +4170,7 @@ static void keys_task (int ch) {
       case 'n':
          if (VIZCHKw(w)) {
             int num = get_int(fmtmk(N_fmt(GET_max_task_fmt), w->rc.maxtasks));
-            if (INT_MIN < num) {
+            if (GET_INTNONE < num) {
                if (-1 < num ) w->rc.maxtasks = num;
                else show_msg(N_txt(BAD_max_task_txt));
             }