refactor: Make prune_gc_sanctions_list more obviously correct.

`target_ext_pk` is uninitialised before the loop, and is definitely
initialised after the loop *iff* `sanction` isn't NULL. If it is NULL,
then `target_ext_pk` is not read. This is verifiable, but not obvious.
PVS Studio isn't smart enough to figure that out, so it complains. I
consider it a valid complaint, and the new code is clearer: we now call
the processing part of the function directly with initialised values
instead of relying on that initialisation coinciding with `sanction`s
nullity.

toxcore/group_chats.c

@@ -1086,43 +1086,16 @@ static bool prune_gc_mod_list(GC_Chat *chat)
            && update_gc_topic(chat, public_sig_key);
 }
 
-/** @brief Removes the first found offline sanctioned peer from the sanctions list and sends the
- * event to the rest of the group.
- *
- * @retval false on failure or if no presently sanctioned peer is offline.
- */
 non_null()
-static bool prune_gc_sanctions_list(GC_Chat *chat)
+static bool prune_gc_sanctions_list_inner(
+    GC_Chat *chat, const Mod_Sanction *sanction,
+    const uint8_t target_ext_pk[ENC_PUBLIC_KEY_SIZE + SIG_PUBLIC_KEY_SIZE])
 {
-    if (chat->moderation.num_sanctions == 0) {
-        return true;
-    }
-
-    const Mod_Sanction *sanction = nullptr;
-    uint8_t target_ext_pk[ENC_PUBLIC_KEY_SIZE + SIG_PUBLIC_KEY_SIZE];
-
-    for (uint16_t i = 0; i < chat->moderation.num_sanctions; ++i) {
-        const int peer_number = get_peer_number_of_enc_pk(chat, chat->moderation.sanctions[i].target_public_enc_key, true);
-
-        if (peer_number == -1) {
-            sanction = &chat->moderation.sanctions[i];
-            memcpy(target_ext_pk, sanction->target_public_enc_key, ENC_PUBLIC_KEY_SIZE);
-            memcpy(target_ext_pk + ENC_PUBLIC_KEY_SIZE, sanction->setter_public_sig_key, SIG_PUBLIC_KEY_SIZE);
-            break;
-        }
-    }
-
-    if (sanction == nullptr) {
-        return false;
-    }
-
     if (!sanctions_list_remove_observer(&chat->moderation, sanction->target_public_enc_key, nullptr)) {
         LOGGER_WARNING(chat->log, "Failed to remove entry from observer list");
         return false;
     }
 
-    sanction = nullptr;
-
     uint8_t data[MOD_SANCTIONS_CREDS_SIZE];
     const uint16_t length = sanctions_creds_pack(&chat->moderation.sanctions_creds, data);
 
@@ -1139,6 +1112,33 @@ static bool prune_gc_sanctions_list(GC_Chat *chat)
     return true;
 }
 
+/** @brief Removes the first found offline sanctioned peer from the sanctions list and sends the
+ * event to the rest of the group.
+ *
+ * @retval false on failure or if no presently sanctioned peer is offline.
+ */
+non_null()
+static bool prune_gc_sanctions_list(GC_Chat *chat)
+{
+    if (chat->moderation.num_sanctions == 0) {
+        return true;
+    }
+
+    for (uint16_t i = 0; i < chat->moderation.num_sanctions; ++i) {
+        const int peer_number = get_peer_number_of_enc_pk(chat, chat->moderation.sanctions[i].target_public_enc_key, true);
+
+        if (peer_number == -1) {
+            const Mod_Sanction *sanction = &chat->moderation.sanctions[i];
+            uint8_t target_ext_pk[ENC_PUBLIC_KEY_SIZE + SIG_PUBLIC_KEY_SIZE];
+            memcpy(target_ext_pk, sanction->target_public_enc_key, ENC_PUBLIC_KEY_SIZE);
+            memcpy(target_ext_pk + ENC_PUBLIC_KEY_SIZE, sanction->setter_public_sig_key, SIG_PUBLIC_KEY_SIZE);
+            return prune_gc_sanctions_list_inner(chat, sanction, target_ext_pk);
+        }
+    }
+
+    return false;
+}
+
 /** @brief Size of peer data that we pack for transfer (nick length must be accounted for separately).
  * packed data consists of: nick length, nick, and status.
  */