draw: fix clip test with NaNs

draw: fix clip test with NaNs

NaNs mean it should be clipped, otherwise the NaNs might get passed to the
next stages (if clipping didn't happen for another reason already), which
might cause all kind of problems.
The llvm path got this right already (possibly by luck), but this isn't used
when there's a gs active.
Found by code inspection, verified with some hacked piglit test and some more
hacked debug output.
(Note the clipper can still itself incorrectly generate NaN and INF position
values in its output prims (at least after w divide / viewport transform) even
if the inputs weren't NaNs, if the position data of the vertices is
"sufficiently bad".)

Reviewed-by: Brian Paul <brianp@vmware.com>