Segmentation fault when open project with grdecl-data

[hhgs]

Steps:

1. Create project with grdecl-input data
2. Save and close ResInsight
3. Open ResInsight and open project
   --> Grid surface color has change to 'none', even if same Cell Result parameter is selected
4. Try to change 3D view
   => resinsight201801: line 21: 51115 Minnesegmentsfeil

[magnesj]

Guard RiuResultQwtPlot::addCurve when no data is available

ACTNUM displays no colors in 3D view

[rebcox1]

Probably introduced in #1642