[Kst] [Kst-plot/kst] dc7426: Don't crash on crazy time axis values

[Barth Netterfield]

  Branch: refs/heads/master
  Home:   https://github.com/Kst-plot/kst
  Commit: dc74268da5114e360136ecd620577751b9d3231e
      https://github.com/Kst-plot/kst/commit/dc74268da5114e360136ecd620577751b9d3231e
  Author: Barth Netterfield <netterfield at astro.utoronto.ca>
  Date:   2016-06-11 (Sat, 11 Jun 2016)

  Changed paths:
    M src/libkstapp/plotaxis.cpp

  Log Message:
  -----------
  Don't crash on crazy time axis values

Instead just draw a blank axis.
Crazy is defined as  t>1E150 years.