st/mesa: Validate state before doing blits.
st/mesa: Validate state before doing blits.

Specifically, this ensures things like the front buffer actually exist. This
fixes piglt fbo/fbo-sys-blit and fd.o bug 35483.

Signed-off-by: Henri Verbeet <fa9c81d52bf7e42f4eca779fafc7ac8cc1c787d8@gmail.com>

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