Bugfix: crash in rgbmatrixeditor

- create group of fixtures, e.g 4 dimmers
- create rgb matrix e.g. full columns
- set backward direction
- save workspace
- load workspace
- edit the rgb matrix
- boom!

(on the first run through, m_previewStep is set to -1, because the m_previewMap is still empty)
Merge pull request #188 from janosvitok/rgbmatrix-crash

Bugfix: crash in rgbmatrixeditor