[Kst-plot/kst] 95eed8: Fix crash with --png.

[Barth Netterfield]

  Branch: refs/heads/master
  Home:   https://github.com/Kst-plot/kst
  Commit: 95eed82c287dba51891d4cfdfb97fbb84f5a5cf4
      https://github.com/Kst-plot/kst/commit/95eed82c287dba51891d4cfdfb97fbb84f5a5cf4
  Author: Barth Netterfield <netterfield at astro.utoronto.ca>
  Date:   2016-08-19 (Fri, 19 Aug 2016)

  Changed paths:
    M src/libkstapp/commandlineparser.cpp
    M src/libkstapp/mainwindow.cpp
    M src/libkstapp/mainwindow.h

  Log Message:
  -----------
  Fix crash with --png.

Also print the usage message in release mode.