bytestring: avoid compile_fail failure due to uninitialized warning

bytestring: avoid compile_fail failure due to uninitialized warning

bytestring: Module tests compile (tests_compile): FAIL
/home/x/g/ccc/ccan/ccan/bytestring/test/compile_fail-BYTESTRING-2.c:Compile gave warnings without -DFAIL:
/home/x/g/ccc/ccan/ccan/bytestring/test/compile_fail-BYTESTRING-2.c: In function ‘main’:
/home/x/g/ccc/ccan/ccan/bytestring/test/compile_fail-BYTESTRING-2.c:15:2: warning: ‘bs.len’ is used uninitialized in this function [-Wuninitialized]
  printf("%zd %s\n", bs.len, x);
  ^

Signed-off-by: Cody P Schafer <dev@codyps.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>