lib/charset: utf8_get() should return error

utf8_get() should return an error if hitting an illegal UTF-8 sequence and
not silently convert the input to a question mark.

Correct utf_8() and the its unit test.

console_read_unicode() now will ignore illegal UTF-8 sequences.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>