IP4 class: ensure it really is an IPv4 address

Make sure we don't accidentally get confused by IPv6 addresses
returned by DNS queries.

esp32/max80/IP4.h

@@ -24,8 +24,10 @@ public:
     constexpr IP4(uint8_t b0, uint8_t b1, uint8_t b2, uint8_t b3) :
 	b{b0,b1,b2,b3} { }
     IP4(const IPAddress & ip) : l{ip} { }
-    constexpr IP4(const ip_addr_t & ip) : l{ip.u_addr.ip4.addr} { }
-    constexpr IP4(const esp_ip_addr_t & ip) : l{ip.u_addr.ip4.addr} { }
+    constexpr IP4(const ip_addr_t & ip) :
+	l{ip.type == IPADDR_TYPE_V4 ? ip.u_addr.ip4.addr : 0} { }
+    constexpr IP4(const esp_ip_addr_t & ip) :
+	l{ip.type == IPADDR_TYPE_V4 ? ip.u_addr.ip4.addr : 0} { }
     IP4(const char *str);
 
     operator uint32_t () { return l; }