ncurses 6.0 - patch 20170408

+ change logic in wins_nwstr() to avoid addressing data past the output
  of mbstowcs().
+ correct a call to setcchar() in Data_Entry_w() from 20131207 changes.
+ fix minor memory-leaks in test-programs.
+ further improve ifdef in term_entry.h for internal definitions not
  used by tack.