use an example UUID to define H2O_UUID_STR_RFC4122_LEN
sited from "Appendix B. Sample Output of utest"
/*
* size of a UUID string representation.
*/
-#define H2O_UUID_STR_RFC4122_LEN 36
+#define H2O_UUID_STR_RFC4122_LEN (sizeof("7d444840-9dc0-11d1-b245-5ffdce74fad2") - 1)
/**
* generates and sets a UUIDv4 to buf, which must have an enough size, H2O_UUID_STR_RFC4122_LEN + 1.
