xx 2017.19 released. Some bug fixes for crashes

[sean]

2017.19 released.

This is a small release with bug fixes for some crashes. thanks Dan and Keith for the fixes!

Latest releases are found at www.xlights.org
Videos tutorials are found at videos.xlights.org
Forums are located at nutcracker123.com/forum
Issue Tracker is found here: www.github.com/smeighan/xLights/issues

XLIGHTS/NUTCRACKER RELEASE NOTES:
---------------------------------
2017.19  Jul 08, 2017
   -- bug (dkulp)   Guard against submodels that result in 0 nodes that could
                    create gigantic buffers. Fixes #879
   -- bug (dkulp)   Fix crash with exporting of model group.  Fixes # 878
   -- bug (keith)   Put in place protections against extreme gamma values.
   -- bug (keith)   Fix show all effects crash because dynamic cast failed