Using substr can cut a multi-byte character in half.
Also, if a long post with many multi-byte characters was reported, then
the mod interface would temporarily extend the body_truncate_char
setting to be sure to cover all of the *characters* in the report, but
this function would interpret body_truncate_char as a number of *bytes*,
so sometimes the end of the report's appended html would be cut off.
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