libio: Flush-only _IO_str_overflow must not return EOF (bug 28949)

In general, _IO_str_overflow returns the character passed as an argument
on success.  However, if flush-only operation is requested by passing
EOF, returning EOF looks like an error, and the caller cannot tell
whether the operation was successful or not.

_IO_wstr_overflow had the same bug regarding WEOF.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>