glsl: Initialize add_uniform_to_shader member var in constructor.

Fix defect reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member var is not initialized in
this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7121>

src/mesa/program/ir_to_mesa.cpp

@@ -2336,7 +2336,8 @@ public:
    add_uniform_to_shader(struct gl_context *ctx,
                          struct gl_shader_program *shader_program,
 			 struct gl_program_parameter_list *params)
-      : ctx(ctx), shader_program(shader_program), params(params), idx(-1)
+      : ctx(ctx), shader_program(shader_program), params(params), idx(-1),
+        var(NULL)
    {
       /* empty */
    }