lib/vsprintf: print '?' for illegal Unicode sequence

Commit 0e66c10a ("lib: vsprintf: avoid overflow printing UTF16
strings") broke the Unicode unit tests: an illegal UTF16 code point
should be printed as '?'.

Fixes: 0e66c10a ("lib: vsprintf: avoid overflow printing UTF16 strings")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>