st/nine: Ignore D3DSIO_RET if it is the last instruction in a shader

RET as a last instruction could be safely ignored.
Remove it to prevent crashes/warnings in case underlying driver
doesn't implement arbitrary returns.

A better way would be to remove the RET after the whole shader
is parsed which will handle a possible case when the last RET is
followed by a comment.

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Axel Davy <davyaxel0@gmail.com>
(cherry picked from commit 2d8f77db)