Layer destroy() needs to be wary of double-destroys

[euzuro]

If OpenLayers.Layer.destroy() gets called twice, line 191:

       this.events.destroy();

will raise an error, because this.events will be nulled out.

This might happen in a custom layer which implements more than one Layer superclass, (like OpenLayers.Layer.Markers and OpenLayers.Layer.HTTPRequest, for example), since it should be calling both destroys, and both destroys will call the generic OpenLayers.Layer.destroy()

[euzuro]

be more wary in destroy() of OpenLayers.Layer

[sderle]

no braner. go for it.

[euzuro]

r3023. thx sde

[euzuro]

oops. forgot we're in a release cycle. reopening, marking as 'pullup'

[crschmidt]

Brought up to 2.4 branch for RC2 with r3088