Fix slice bounds out of range on address copy

An IP address embedded in a net.TCPAddr (or net.UDPAddr) struct returned
from net.ResolveTCPAddr (or net.ResolveUDPAddr) can be either a 4-byte
or 16-byte slice depending on platform and configuration. Therefore, we
must check the length of the address before copying it into the
syscall.Sockaddr to avoid a "slice bounds out of range" panic.

tcp.go

@@ -40,7 +40,11 @@ func getTCPSockaddr(proto, addr string) (sa syscall.Sockaddr, soType int, err er
 		sa := &syscall.SockaddrInet4{Port: tcp.Port}
 
 		if tcp.IP != nil {
-			copy(sa.Addr[:], tcp.IP[12:16]) // copy last 4 bytes of slice to array
+			if len(tcp.IP) == 16 {
+				copy(sa.Addr[:], tcp.IP[12:16]) // copy last 4 bytes of slice to array
+			} else {
+				copy(sa.Addr[:], tcp.IP) // copy all bytes of slice to array
+			}
 		}
 
 		return sa, syscall.AF_INET, nil

udp.go

@@ -37,7 +37,11 @@ func getUDPSockaddr(proto, addr string) (sa syscall.Sockaddr, soType int, err er
 		sa := &syscall.SockaddrInet4{Port: udp.Port}
 
 		if udp.IP != nil {
-			copy(sa.Addr[:], udp.IP[12:16]) // copy last 4 bytes of slice to array
+			if len(udp.IP) == 16 {
+				copy(sa.Addr[:], udp.IP[12:16]) // copy last 4 bytes of slice to array
+			} else {
+				copy(sa.Addr[:], udp.IP) // copy all bytes of slice to array
+			}
 		}
 
 		return sa, syscall.AF_INET, nil