curses: Pass in proper NULL pointer
curses: Pass in proper NULL pointer

Fix sparse warning:
out_curses.c:433:25: warning: Using plain integer as NULL pointer

Signed-off-by: Thomas Graf <17c2d8920621522552b392f72d8fdb4d1e866b44@suug.ch>

tklauser/bmon,Chocobo1/bmon,Chocobo1/bmon,tklauser/bmon