Fix Bug #10238: Display filter expression dialog items do not expand/display properly.

    Unfortunately, certain proto_hier_tree_model.c functions
    assume/require that a cookie generated by
    proto_(first|next)_protocol_field() will never have a NULL value.

    Bug introduced in gd47ae54.

Change-Id: I42763d02f700e15ca9b3ab9980943d4f8d933ca9
Reviewed-on: https://code.wireshark.org/review/2712
Reviewed-by: Evan Huus <eapache@gmail.com>