lib: vsprintf: avoid overflow printing UTF16 strings

We have to ensure while printing UTF16 strings that we do not exceed the
end of the print buffer.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>