glsl: Initialize coordinate to NULL in ir_texture constructor.

glsl: Initialize coordinate to NULL in ir_texture constructor.

I ran into this while trying to create a TXS query, which doesn't have a
coordinate.  Since it didn't get initialized to NULL, a bunch of
visitors tried to access it and crashed.

Most of the time, this won't be a problem, but it's just a good idea.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>