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

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 39f4067c66141921a9ca2a5170df1fa2151f3182

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