glsl: fixed uninitialized pointer

glsl: fixed uninitialized pointer

Class "ir_constant" had a bunch of constructors where the pointer member
"array_elements" had not been initialized. This could have lead to unsafe
code if something had tried to write anything to it. This patch fixes
this issue by initializing the pointer to NULL in all the constructors.
This issue was discovered by Coverity.

CID: 401603, 401604, 401605, 401610

Signed-off-by: Jakob Sinclair <sinclair.jakob@openmailbox.org>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>