[patch] libstdc++/64646 fix 4-iterator overload of std::is_permutation

[Jonathan Wakely]

Fix a past-the-end dereference in the new C++14 version of
is_permutation. This only affects C++14 mode, but fixes a new function
that exists entirely because it's meant to be safer than the C++98
version ... but it isn't safer if it goes past-the-end!

Tested x86_64-linux, committed to 4.9 and trunk.
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