opengl/eglcontext: only delete the vao if the context is current

Xaver Huglrequested to merge
work/zamundaaa/delete-vao into master

Making the context can fail during a GPU reset, and then we'd crash trying to access the OpenGL function.

This fixes https://crash-reports.kde.org/share/issue/425cb2a56d6d4cdeadfc9e985726a7e8/

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