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>

ccan/bytestring/test/compile_fail-BYTESTRING-2.c

@@ -11,6 +11,8 @@ int main(int argc, char *argv[])
 
 #ifdef FAIL
 	bs = BYTESTRING(x);
+#else
+	bs.len = 0;
 #endif
 	printf("%zd %s\n", bs.len, x);
 	return 0;