Fixed segfault with filter_finalize introduced in r4621 (1.3.0).

Fixed segfault with filter_finalize introduced in r4621 (1.3.0).

Example configuration to reproduce:

    location /image/ {
        error_page 415 = /zero;
        image_filter crop 100 100;
        proxy_pass http://127.0.0.1:8080;
        proxy_store on;
    }

    location /zero {
        return 204;
    }

The problem appeared if upstream returned (big enough) non-image file,
causing 415 to be generated by image filter.