flacenc: avoid potential string overflow

flacenc: avoid potential string overflow

We don't necessarily have full control over the input tags, so
it's possible that the ISRC tag contains a longer string than
expected, in which case we'd write over the end of the static-size
13 byte buffer that is FLAC__StreamMetadata_CueSheet_Track::isrc.
Make sure to only copy the ISRC if it's not too long, and make
sure the buffer we write to is always NUL-terminated by using
g_strlcpy().

CID 1324931.