Insert Addressing::LeaveGroup call on dropping multicast group membership.

Omitting this call left invalid reference to deleted group in the
multicast groups hash-map and any upcoming attempt to join the group will
crash the system. Fixes #6736.

src/add-ons/kernel/network/protocols/ipv4/multicast.cpp

@@ -61,6 +61,7 @@ template<typename Addressing> status_t
 MulticastGroupInterface<Addressing>::Drop()
 {
 	fAddresses.Clear();
+	Addressing::LeaveGroup(this);
 	fFilterMode = kInclude;
 	return B_OK;
 }

src/add-ons/kernel/network/protocols/ipv6/multicast.cpp

@@ -50,6 +50,7 @@ template<typename Addressing> status_t
 MulticastGroupInterface<Addressing>::Drop()
 {
 	fAddresses.Clear();
+	Addressing::LeaveGroup(this);
 	fFilterMode = kInclude;
 	return B_OK;
 }