Properly accept wildcard when binding IPv4 socket.

Also never return null for is_ipv4 to avoid crashes due to engine bug.
(better to get an error and a broken socket then seeing your game crash)

core/io/ip_address.cpp

@@ -184,7 +184,7 @@ bool IP_Address::is_ipv4() const {
 }
 
 const uint8_t *IP_Address::get_ipv4() const {
-	ERR_FAIL_COND_V(!is_ipv4(), 0);
+	ERR_FAIL_COND_V(!is_ipv4(), &(field8[12])); // Not the correct IPv4 (it's an IPv6), but we don't want to return a null pointer risking an engine crash.
 	return &(field8[12]);
 }