libio: Check remaining buffer size in _IO_wdo_write (bug 31183)

The multibyte character needs to fit into the remaining buffer space,
not the already-written buffer space.  Without the fix, we were never
moving the write pointer from the start of the buffer, always using
the single-character fallback buffer.

Fixes commit 04b76b5a ("Don't error out writing
a multibyte character to an unbuffered stream (bug 17522)").

(cherry picked from commit ecc7c3de)