st/nine: Handle NRM with input of null norm

st/nine: Handle NRM with input of null norm

When the input's xyz are 0.0, the output
should be 0.0. This is due to the fact that
Inf * 0 = 0 for dx9. To handle this case,
cap the result of RSQ to FLT_MAX. We have
FLT_MAX * 0 = 0.

Reviewed-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Axel Davy <axel.davy@ens.fr>
Cc: "10.4" <mesa-stable@lists.freedesktop.org>