Do not take address of empty string front

[François Dumont]

Hi

    2 experimental tests are failing in debug mode because
__do_str_codecvt is sometimes taking address of string front() and
back() even if empty. It wasn't use so not a big issue but it still
seems better to avoid. I propose to rather use string begin() to get
buffer address. I kept operator& as it is what was used, do you think we
should use addressof ?

    At the same time I improve string debug mode cause with front()
implemented as operator[](0) it is ok even if string is empty while
back() implemented as operator[](size()-1) is not which is quite
inconsistent. So I added a number of !empty() checks in several methods
to detect more easily all those invalid usages.

    * include/bits/basic_string.h (basic_string<>::front()): Add !empty
    debug check.
    (basic_string<>::back()): Likewise.
    (basic_string<>::pop_back()): Likewise.
    * include/bits/locale_env.h (__do_std_codecvt): Do not take address of
    string::front() when string is empty.
    * include/debug/macros.h
    (__glibcxx_check_string, __glibcxx_check_string_len): Implement using
    _GLIBCXX_DEBUG_PEDASSERT.

Tested under Linux x86_64 debug mode.

Ok to commit ?

François

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