Fix NRE Panic in dialSCTPExtConfig and listenSCTPExtConfig when control is used

[paulrozhkin]

Description

This PR fixes a nil reference panic that occurs in dialSCTPExtConfig and listenSCTPExtConfig when the control function is passed and laddr is nil (SCTP use without SCTP_SOCKOPT_BINDX_ADD).
The issue was caused by calling laddr.String() without checking for nil.

Panic Stacktrace:

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=0x0 pc=0x53be03]

goroutine 6 [running]:
testing.tRunner.func1.2({0x55b2a0, 0x695fc0})
	/usr/local/go/src/testing/testing.go:1526 +0x24e
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1529 +0x39f
panic({0x55b2a0, 0x695fc0})
	/usr/local/go/src/runtime/panic.go:884 +0x213
github.com/ishidawataru/sctp.(*SCTPAddr).String(0x0)

Fix

Added a nil check before accessing laddr.String() in both functions. Pass an empty string to the control function:

var localAddressString string  
if laddr != nil {  
    localAddressString = laddr.String()  
}
if err = control(network, localAddressString, rc); err != nil {
    return nil, err
}

[ishidawataru]

Thanks!