connectd: fix valgrind complaint on FreeBSD

As reported by Wladimir J. van der Laan.  Valgrind will complain
about padding and unset fields, so memset the structs.

```
==42653== Syscall param socketcall.connect(serv_addr..sa_len) points to uninitialised byte(s)
==42653==    at 0x4C7D19A: _connect (in /lib/libc.so.7)
==42653==    by 0x4EE1F35: ??? (in /lib/libthr.so.3)
==42653==    by 0x249D57: get_local_sockname (netaddress.c:212)
==42653==    by 0x249CDB: guess_address (netaddress.c:242)
==42653==    by 0x2473D0: public_address (connectd.c:1003)
==42653==    by 0x246CE4: setup_listeners (connectd.c:0)
==42653==    by 0x246566: connect_init (connectd.c:1311)
==42653==    by 0x270CEB: next_plan (io.c:59)
==42653==    by 0x2713EE: io_ready (io.c:417)
==42653==    by 0x2726B1: io_loop (poll.c:445)
==42653==    by 0x24618A: main (connectd.c:1703)
==42653==  Address 0x7fc000690 is on thread 1's stack
==42653==  in frame #3, created by guess_address (netaddress.c:231)
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

connectd/netaddress.c

@@ -235,6 +235,7 @@ bool guess_address(struct wireaddr *addr)
     switch (addr->type) {
     case ADDR_TYPE_IPV4: {
         struct sockaddr_in sin;
+        memset(&sin, 0, sizeof(sin));
 	sin.sin_port = htons(53);
         /* 8.8.8.8 */
 	sin.sin_addr.s_addr = 0x08080808;
@@ -246,6 +247,7 @@ bool guess_address(struct wireaddr *addr)
     }
     case ADDR_TYPE_IPV6: {
         struct sockaddr_in6 sin6;
+        memset(&sin6, 0, sizeof(sin6));
         /* 2001:4860:4860::8888 */
         static const unsigned char pchGoogle[16]
                 = {0x20,0x01,0x48,0x60,0x48,0x60,0,0,0,0,0,0,8,8,8,8};