Erase correctly math multi-cell selection

What a selection spans several cells in mathed, and a deletion occurs though LFUN_CHAR_DELETE_FORWARD for example, only the case of a proper grid was handled. When no such grid exists, all cells between the first and the last index are cleared now.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.10.16-15.00