glsl: Initialize assignment_generator member variables.

Fixes "Uninitialized pointer field" defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/lower_variable_index_to_cond_assign.cpp

@@ -192,6 +192,12 @@ struct assignment_generator
    ir_variable* var;
 
    assignment_generator()
+      : base_ir(NULL),
+        rvalue(NULL),
+        old_index(NULL),
+        is_write(false),
+        write_mask(0),
+        var(NULL)
    {
    }