Added additional check in UpdateRowColDefinitions to avoid exception.

[bdachev]

With certain layouts (produced by our software) undocking an anchorable leads to following:

System.ArgumentOutOfRangeException
HResult=0x80131502
Message=Specified argument was out of the range of valid values.
Parameter name: index
Source=PresentationCore
StackTrace:
at System.Windows.Media.VisualCollection.get_Item(Int32 index)
at System.Windows.Controls.UIElementCollection.get_Item(Int32 index)
at AvalonDock.Controls.LayoutGridControl`1.UpdateRowColDefinitions() in C:\SOURCE\Tests\AvalonDock\Dirkster99\source\Components\AvalonDock\Controls\LayoutGridControl.cs:line 255

This exception was originally thrown at this call stack:
[External Code]
AvalonDock.Controls.LayoutGridControl.UpdateRowColDefinitions() in LayoutGridControl.cs