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 "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).