Be explicit about truncation to silence warning.

Be explicit about truncation to silence warning.

Newer GCCs do not like truncation and emit a warning.  We don't want to
disable truncation warnings (-Wnostringop-truncation), as in some cases
these warnings can point to a security flaw.

Using strcat instead of strncat is fine, since *both* equally overflow
the destination buffer if not used carefully.

See https://developers.redhat.com/blog/2018/05/24/detecting-string-truncation-with-gcc-8/