glsl: propagate errors from *=, /=, +=, -= operators

Fixes compiler crash on:

void main()
{
    gl_FragColor = a += 1;
}

(a is not declared anywhere)

Found with AFL++.

Fixes: d1fa69ed61 ("glsl: do not attempt assignment if operand type not parsed correctly")
Signed-off-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12717>

src/compiler/glsl/ast_to_hir.cpp

@@ -1703,6 +1703,7 @@ ast_expression::do_hir(exec_list *instructions,
       if ((op[0]->type == glsl_type::error_type ||
            op[1]->type == glsl_type::error_type)) {
          error_emitted = true;
+         result = ir_rvalue::error_value(ctx);
          break;
       }