glsl: assert base_alignment > 0 for records

glsl: assert base_alignment > 0 for records

From GLSL 1.50 spec, section 4.1.8 "Structures":

"Structures must have at least one member declaration."

So the base_alignment should be higher than zero.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>