Search snippets can be cut inside a multi-byte character

With hl.fragsize=200 the highlight fragment of emoji-rich content ends in U+FFFD replacement characters. (ex:  🚀🌟🔥🎉✨🌈🍀🍎���....)

Expected: The fragment ends on a character (ideally word) boundary.

Actual: Three replacement characters before the ellipsis.