fix uninitialized read when stringifying an addrLocal

Reachable from either place where SetIP is used when our best-guess
addrLocal for a peer is IPv4, but the peer tells us it's reaching us at
an IPv6 address.

In that case, SetIP turns an IPv4 address into an IPv6 address without
setting the scopeId, which is subsequently read in GetSockAddr during
CNetAddr::ToStringIP and passed to getnameinfo. Fix by ensuring every
constructor initializes the scopeId field with something.

src/netaddress.cpp

@@ -17,7 +17,6 @@ static const unsigned char g_internal_prefix[] = { 0xFD, 0x6B, 0x88, 0xC0, 0x87,
 CNetAddr::CNetAddr()
 {
     memset(ip, 0, sizeof(ip));
-    scopeId = 0;
 }
 
 void CNetAddr::SetIP(const CNetAddr& ipIn)

src/netaddress.h

@@ -33,7 +33,7 @@ class CNetAddr
 {
     protected:
         unsigned char ip[16]; // in network byte order
-        uint32_t scopeId; // for scoped/link-local ipv6 addresses
+        uint32_t scopeId{0}; // for scoped/link-local ipv6 addresses
 
     public:
         CNetAddr();