ncurses 6.1 - patch 20180217

+ remove incorrect free() from 20170617 changes (report by David Macek).
+ correct type for "U8" in user_caps.5; it is a number not boolean.
+ add a null-pointer check in safe_sprintf.c (report by Steven Noonan).
+ improve fix for Debian #882620 by reusing limit2 variable (report
  Julien Cristau, Sven Cristau).