Effects: Behaves according to the description of
basic_streambuf<charT, traits>::underflow(),
with the specialization that a sequence of characters is read from the input
sequence as if by reading from the associated file
into an internal buffer (extern_buf)
and then as if by doing:
char extern_buf[XSIZE];
char* extern_end;
charT intern_buf[ISIZE];
charT* intern_end;
codecvt_base::result r =
a_codecvt.in(state, extern_buf, extern_buf+XSIZE, extern_end,
intern_buf, intern_buf+ISIZE, intern_end);