i830: Fix crash for GL_STENCIL_TEST in i830Enable()
i830: Fix crash for GL_STENCIL_TEST in i830Enable()

commit 87f12bb2d95236c7b025d1a8be56b5ab1683d702 tried to fix rb->mt
being NULL, but change this case wrong.

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Kurt Roeckx <bb87b47479d83cec3c76132206933257ded727b2@roeckx.be>
Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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