softpipe: Fix segfault with fbo-cubemap.

softpipe: Fix segfault with fbo-cubemap.

The cube sampler generates two-dimensional texture coordinates and
hence passes NULL for the array for the third one.  The actual 2D
sampler, lower in the pipe, knew not to used that array since it
didn't need it.  But the samplers have become single-texel and the
coordinate array dereference has been moved up one step, to a level
where the code does not know only two coordinates are used.  Hence the
segfault.

The simplest fix by far is to add a third dummy coordinate array in
the call to the next pipe step, which will be dereferenced to an
harmless 0 which then will be happily ignored by the sampler.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=52250

Signed-off-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Brian Paul <brianp@vmware.com>