create propper constructor for Group_Peer and fix crash

The crash happened, because the 'free_peer_members' function didn't set
'nick_len = 0'

toxcore/group.c

@@ -609,6 +609,13 @@ static void apply_changes_in_peers(Group_Chats *g_c, int32_t groupnumber, void *
         g->dirty_list = false;
     }
 
+    // TODO(sudden6): remove this crow-bar assertion
+    // ensure no ALMOST_DELETED_PEER peers are in the peer list
+    for (uint32_t i = 0; i < g->numpeers_in_list; ++i) {
+        Group_Peer *peer = &g->peers[g->peers_list[i]];
+        assert(peer->friendcon_id != ALMOST_DELETED_PEER);
+    }
+
     /* notify client about nick change */
     if (g->nick_changed) {
         for (uint32_t i = 0; i < g->numpeers_in_list; ++i) {
@@ -806,6 +813,38 @@ static void need_send_peers(Group_c *g)
     }
 }
 
+static void init_group_peer(Group_Peer *p, const uint8_t *real_pk, const uint8_t *temp_pk, int peer_gid)
+{
+    /* Group_Peer constructor */
+
+    p->lossy = nullptr;
+    p->object = nullptr;
+    p->nick = nullptr;
+    p->nick_len = 0;
+    p->keep_connection = 0;
+    p->nick_changed = 0;
+    p->title_changed = 0;
+    p->auto_join = 0;
+    p->need_send_peers = 0;
+    p->connected = 0;
+
+    /* set undefined */
+    p->friendcon_id = -1;
+
+    id_copy(p->real_pk, real_pk);
+    id_copy(p->temp_pk, temp_pk);
+
+    if (peer_gid >= 0) {
+        p->gid = peer_gid;
+    } else {
+        p->gid = -1;
+    }
+
+    // invalid by default
+    p->group_number = INVALID_GROUP_NUMBER;
+    p->last_recv = unix_time();
+}
+
 static int64_t addpeer(Group_c *g, int32_t groupnumber, const uint8_t *real_pk, const uint8_t *temp_pk, int peer_gid)
 {
     if (peer_gid >= 0) {
@@ -847,31 +886,20 @@ static int64_t addpeer(Group_c *g, int32_t groupnumber, const uint8_t *real_pk,
     }
 
     g->peers = new_peers;
-    new_peers = new_peers + g->numpeers;
+
+    Group_Peer *new_peer = &new_peers[g->numpeers];
     ++g->numpeers;
 
     /* Group_Peer constructor */
-
-    memset(new_peers, 0, sizeof(Group_Peer));
-
-    /* set undefined */
-    new_peers->friendcon_id = -1;
+    init_group_peer(new_peer, real_pk, temp_pk, peer_gid);
 
     /* set undefined for other */
-    new_peers->group_number = id_equal(g->real_pk, real_pk) ? (uint16_t)groupnumber : INVALID_GROUP_NUMBER;
-
-    id_copy(new_peers->real_pk, real_pk);
-    id_copy(new_peers->temp_pk, temp_pk);
+    new_peer->group_number = id_equal(g->real_pk, real_pk) ? (uint16_t)groupnumber : INVALID_GROUP_NUMBER;
 
     if (peer_gid >= 0) {
         need_send_peers(g);
-        new_peers->gid = peer_gid;
-    } else {
-        new_peers->gid = -1;
     }
 
-    new_peers->last_recv = unix_time();
-
     g->dirty_list = true;
 
     if (g->peer_on_join) {
@@ -886,6 +914,7 @@ static void free_peer_members(Group_Peer *peer)
     free(peer->lossy);
     peer->lossy = nullptr;
 
+    peer->nick_len = 0;
     free(peer->nick);
     peer->nick = nullptr;
 }
@@ -952,6 +981,8 @@ static bool setnick(Group_c *g, int64_t peer_index, const uint8_t *nick, uint8_t
 
     Group_Peer *peer = &g->peers[peer_index];
 
+    assert(peer->friendcon_id != ALMOST_DELETED_PEER);
+
     /* same name as already stored? */
     if (peer->nick_len == nick_len && memcmp(peer->nick, nick, nick_len) == 0) {
         return true;
@@ -1275,6 +1306,10 @@ int group_peername(const Group_Chats *g_c, int groupnumber, int peer_index, uint
 
     Group_Peer *peer = &g->peers[g->peers_list[peer_index]];
 
+    if (peer->friendcon_id == ALMOST_DELETED_PEER) {
+        return  -2;
+    }
+
     peer->nick_changed = false;
 
     memcpy(name, peer->nick, peer->nick_len);