Fix crash (div by 0) introduced in r3295, updates #953.

(Blind fix, doesn't happen on Windows.)

Originally committed to SVN as r3297.

aegisub/src/base_grid.cpp

@@ -66,6 +66,7 @@ BaseGrid::BaseGrid(wxWindow* parent, wxWindowID id, const wxPoint& pos, const wx
 	bmp = NULL;
 	holding = false;
 	byFrame = false;
+	lineHeight = 1; // non-zero to avoid div by 0
 
 	// Set scrollbar
 	scrollBar = new wxScrollBar(this,GRID_SCROLLBAR,wxDefaultPosition,wxDefaultSize,wxSB_VERTICAL);