refactor SCTPAddr.String

[ishidawataru]

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

[AkihiroSuda]

Flaky test:

--- FAIL: TestSCTPCloseRecv (1.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x479ddc]
goroutine 284 [running]:
panic(0x54c140, 0xc420016170)
	/home/travis/.gimme/versions/go1.7.linux.amd64/src/runtime/panic.go:500 +0x1a1
testing.tRunner.func1(0xc420534240)
	/home/travis/.gimme/versions/go1.7.linux.amd64/src/testing/testing.go:579 +0x25d
panic(0x54c140, 0xc420016170)
	/home/travis/.gimme/versions/go1.7.linux.amd64/src/runtime/panic.go:458 +0x243
github.com/ishidawataru/sctp.TestSCTPCloseRecv(0xc420534240)
	/home/travis/gopath/src/github.com/ishidawataru/sctp/sctp_test.go:153 +0x27c
testing.tRunner(0xc420534240, 0x588830)
	/home/travis/.gimme/versions/go1.7.linux.amd64/src/testing/testing.go:610 +0x81
created by testing.(*T).Run
	/home/travis/.gimme/versions/go1.7.linux.amd64/src/testing/testing.go:646 +0x2ec
FAIL	github.com/ishidawataru/sctp	1.594s
?   	github.com/ishidawataru/sctp/example	[no test files]

@ishidawataru How does it differ from #10 ?

[AkihiroSuda]

addressed race in #14