[Patch] Fix some problems with _M_extract_via_format

[Paolo Carlini]

Hi,

these are very long standing problems:

  1- At the beginning we were assigning to the char __c
     an element of __format[], an array of _CharTs.

  2- At the end we were comparing a narrowed *__beg to
     the char __c above. This branch is taken when in
     fact __c != '%', therefore, the corresponding
     __format[] element can be any possible char!

The first issue by itself is mostly about type-correctness:
it's unlikely to give problems in practice since __c is
first compared to values in the basic execution character
set (e.g., 'a', 'A', 'h', 'b', ...)

The latter impacts more heavily actual code, as soon as
a wchar_t __format actually contains "exotic" characters,
I have prepared a testcase for this.

Tested x86-linux.

Paolo.

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