Reset safe iterator state only if needed

[François Dumont]

We reset safe iterator state a lot, quite annoying when debugging.

Comitted as trivial.

2018-06-07  François Dumont  <fdumont@gcc.gnu.org>

 Â Â Â  * src/c++11/debug.cc
 Â Â Â  (_Safe_iterator_base::_M_detach()): Reset state only if needed.
 Â Â Â  (_Safe_iterator_base::_M_detach_single()): Likewise.
 Â Â Â  (_Safe_local_iterator_base::_M_detach()): Reset state only if needed.
 Â Â Â  (_Safe_local_iterator_base::_M_detach_single()): Likewise.

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