Per C99, this code was always wrong. The results of `snprintf` are
undefined if the target string array and one of the supplied input
arguments refer to the same buffer. In practice, this particular usage
happened to work by wastefully copying a string onto itself, then adding
more content at the end. Switch to appending properly, which also fixes
the gcc-8 -Wrestrict warning.
References: <https://github.com/dxx-rebirth/dxx-rebirth/issues/388>
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