Apparently the curses on SunOS is less forgiving than the one on GNU.
Or rather: delwin(NULL) should just return an error, it shouldn't crash.

This fixes https://savannah.gnu.org/bugs/?51053

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Reported-by: John Wiersba <jrw32982@yahoo.com>
Solved-by: John Wiersba <jrw32982@yahoo.com>