BLI_strncpy_utf8 didn't check for null bytes within bytes stepped
over by the variable length UTF8 encoding.
While a valid UTF8 string wont include these, it's possible Latin1
encoding or a truncated string includes such characters.
In this case, the entire string is copied as it's not the purpose of
this function to correct or strip invalid/truncated encoding,
only to prevent it from happening in the first place.
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