gh-152503: Fix garbage text from curses wide-character cell reads (GH-152505)

gh-152503: Fix garbage text from curses wide-character cell reads (GH-152505)

window.in_wch(), window.in_wchstr() and window.getbkgrnd() read a cell
into an uninitialized cchar_t, relying on the curses library to leave the
text NUL-terminated -- which ncurses does but X/Open does not require, so
some libraries (such as NetBSD curses) returned uninitialized bytes as
wide characters.  Zero-initialize the cell buffers before the read and
default the getcchar() output to an empty string.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>