Fix some _GLIBCXX_DEBUG pretty printer errors

[François Dumont]

On 08/03/2018 19:11, Jonathan Wakely wrote:
> This caused a new regression for:
>
>    std::forward_list<std::string> flst;
>    std::forward_list<std::string>::iterator flstiter0;
> // { dg-final { note-test flstiter0 {non-dereferenceable iterator for
> std::forward_list}} }
>
>
> $1 = {_M_node = 0x0}
> got: $1 = {_M_node = 0x0}
> FAIL: libstdc++-prettyprinters/debug_cxx11.cc print flstiter0
>
Fixed by the attached patch, committed as trivial.

François

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