avio_put_str16le: Print error message in case of invalid UTF8 input

Found-by: Stefano Sabatini
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/aviobuf: return error from avio_put_str16le() for invalid input

Found-by: Stefano Sabatini
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>