Merge #14728: fix uninitialized read when stringifying an addrLocal

b7b36decaf fix uninitialized read when stringifying an addrLocal (Kaz Wesley)
8ebbef0169 add test demonstrating addrLocal UB (Kaz Wesley)

Pull request description:

  Reachable from either place where SetIP is used when all of:
  - our best-guess addrLocal for a peer is IPv4
  - the peer tells us it's reaching us at an IPv6 address
  - NET logging is enabled

  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.

Tree-SHA512: 8f0159750995e08b985335ccf60a273ebd09003990bcf2c3838b550ed8dc2659552ac7611650e6dd8e29d786fe52ed57674f5880f2e18dc594a7a863134739e3

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();