This is to differentiate between the upcoming `AllocatingMemoryStream`,
which automatically allocates memory as needed instead of operating on a
static memory area.
The round trip compress test wants the first half of the byte buffer to
be filled with random data, and the second half to be all zeroes. The
strategy of using memset on ByteBuffer::offset_pointer confuses
__builtin_memset_chk when building with -fsanitize=undefined. It thinks
that the buffer is using inline capacity when we can prove to ourselves
pretty easily that it's not. To avoid this, just create the buffer
zeroed to start, and then fill the first half with the random data.
