The function message_count_unread_messages counts sent messages

message_count_unread_messages (used in course overview block) gives wrong message count when only one user is supplied, because it counts even sent messages. If both users are supplied, it works correctly.

This function was previously used by Course overview legacy block, now it's gone. We're still using it in our installation, and i'm unsure if it's currently used by any other plugin. But the described function misbehaves anyway.