make OverviewMap.destroy more robust

[elemoine]

the OverviewMap control may produce JavaScript errors when it's destroyed. This happens if the control is destroyed while its update has never been called.

update is called by draw if map.getExtent doesn't return null. If the map doesn't have layers when draw is called then update is not called and the control cannot be destroyed.

[elemoine]

the second paragraph of the ticket description isn't entirely correct. It is correct that draw calls update only if map.getExtent doesn't return null. But this doesn't actually occur in the case where draw is called while the map doesn't have layers (indeed, this particular case is dealt with at the beginning of the draw method). However map.getExtent does return null when the map has no center.

So this sequence produces a JavaScript error:

map = new OpenLayers.Map('map'); 
map.addLayer(new OpenLayers.Layer("layer", {isBaseLayer: true})); 
control = new OpenLayers.Control.OverviewMap(); 
map.addControl(control); 
control.destroy();

(see test-2430.diff )

[elemoine]

patch-2430-r9955-A0.diff fixes it. Tests pass in FF3. Please review.

[fredj]

Please commit (tests and fix)

[elemoine]

fredj, thanks for the review. I'm not committing the test since it doesn't test anything, I had attached it just to show how to reproduce the bug.

[elemoine]

(In [9957]) make OverviewMap.destroy more robust, r=fredj (closes #2430)