Since GCC 8.1, the compiler performs some bounds checking when
copying static data into arrays with a known size. In one test,
we print a format string of "%s/sub%02d" into a buffer of 64
bytes. The input buffer for the first "%s" is bounded to at most
63 characters, plus four bytes for the static string "/sub" plus
two more bytes for "%02d". Thus, our target buffer needs to be at
least 70 bytes in size, including the NUL byte. There seems to be
a bug in the analysis, though, because GCC will not account for
the limiting "%02" prefix, treating it as requiring the same
count of bytes as a "%d".

Thus, we end up at 79 bytes that are required to fix the
warning. To make it look nicer and less special, we just round
the buffer size up to 80 bytes.