Fix safe iterator inconsistent assertion

[François Dumont]

Working on iterator == operator I noticed that a comparison in 
_Safe_iterator was inconsistent.

 Â Â Â  * include/debug/debug.h
 Â Â Â  (_Safe_iterator<>(const _Safe_iterator<_MutableIterator,>& __x)):
 Â Â Â  Compare __x base iterator with a default initialized iterator of the
 Â Â Â  same type.

Tested under linux x86_64.

François

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