refactor SCTPAddr.String

[AkihiroSuda]

Signed-off-by: Akihiro Suda suda.akihiro@lab.ntt.co.jp

[AkihiroSuda]

Flaky test on Go 1.6?

=== RUN   TestStreams
panic: runtime error: index out of range
goroutine 120 [running]:
panic(0x5d42a0, 0xc820014150)
	/home/travis/.gimme/versions/go1.6.linux.amd64/src/runtime/panic.go:464 +0x3e6
math/rand.(*rngSource).Int63(0xc82008e000, 0x1f844ba385a6eb7a)
	/home/travis/.gimme/versions/go1.6.linux.amd64/src/math/rand/rng.go:243 +0xcf
math/rand.(*Rand).Int63(0xc8200149b0, 0x1f844ba385a6eb7a)
	/home/travis/.gimme/versions/go1.6.linux.amd64/src/math/rand/rand.go:46 +0x39
math/rand.(*Rand).Int31(0xc8200149b0, 0xc81f844ba3)
	/home/travis/.gimme/versions/go1.6.linux.amd64/src/math/rand/rand.go:52 +0x21
math/rand.(*Rand).Int31n(0xc8200149b0, 0x3e, 0xc800000023)
	/home/travis/.gimme/versions/go1.6.linux.amd64/src/math/rand/rand.go:87 +0xc1
math/rand.(*Rand).Intn(0xc8200149b0, 0x3e, 0x23)
	/home/travis/.gimme/versions/go1.6.linux.amd64/src/math/rand/rand.go:101 +0x92
github.com/ishidawataru/sctp.TestStreams.func1(0x78, 0x0, 0x0)
	/home/travis/gopath/src/github.com/ishidawataru/sctp/sctp_streams_test.go:23 +0x9e
github.com/ishidawataru/sctp.TestStreams.func3(0xc82001c360, 0xc820032038, 0xc82007c090, 0xc8200149c0, 0xc8200149b0, 0x70)
	/home/travis/gopath/src/github.com/ishidawataru/sctp/sctp_streams_test.go:91 +0x3f5
created by github.com/ishidawataru/sctp.TestStreams
	/home/travis/gopath/src/github.com/ishidawataru/sctp/sctp_streams_test.go:124 +0x4d9
FAIL	github.com/ishidawataru/sctp	0.424s
?   	github.com/ishidawataru/sctp/example	[no test files]

[AkihiroSuda]

The rand panic above is unrelated to this PR. golang/go#21099

[AkihiroSuda]

opened #12

[AkihiroSuda]

rebased