rasterizer crash (patch & testcase)

Both the trunk and mesa-3-5-branch crash in the
software rasterizer for similar reasons.

In the trunk, gl_texture_pixels does not check to see
if lambda is NULL before adding LodBias or clamping
with MinLod and MaxLod.

On the branch, _swrast_texture_pixels does not check to
see if lambda is NULL before adding LodBias. The
clamping loop has already been fixed.The attached file
is a tar containing the patches (utterly trivial) and
two testcases, one for Mesa-3.4.2/DRI trunk and one for
Mesa-3.5/DRI branch.