Prevent calling GL functions with an invalid surface

When destroying CanvasContext, the surface can be invalid
state. So the surface should be updated to null prior to
destroying GL resources to ensure that GL functions
are not called with an invalid surface.
Some GL implementation makes an error if GL functions
are called with an invalid surface. (Adreno 3xx)

Change-Id: Ie6f6ea081ec931fc9df30b2c3ed066ec1ae9d294