libio: Ensure output buffer for wchars (bug #28828)

The _IO_wfile_overflow does not check if the write pointer for wide
data is valid before access, different than _IO_file_overflow.  This
leads to crash on some cases, as described by bug 28828.

The minimal sequence to produce the crash was:

  #include <stdio.h>
  #include <wchar.h>
  int main (int ac, char **av)
  {
    setvbuf (stdout, NULL, _IOLBF, 0);
    fgetwc (stdin);
    fputwc (10, stdout); /*CRASH HERE!*/
    return 0;
  }

The "fgetwc(stdin);" is necessary since it triggers the bug by setting
the flag _IO_CURRENTLY_PUTTING on stdout indirectly (file wfileops.c,
function _IO_wfile_underflow, line 213).

Signed-off-by: Jose Bollo <jobol@nonadev.net>