lib-storage: Update mailbox.recent_flags_count correctly.

The counter was increased even if the UID was already in recent_flags.
This is the only place where I found that recent_flags and
recent_flags_count could become desynced (which they definitely were in
a core dump), so hopefully this fixes assert-crashes like:

Panic: file index-status.c: line 130 (index_storage_get_open_status): assertion failed: (status_r->recent <= status_r->messages)

src/lib-storage/mailbox-recent-flags.c

@@ -23,8 +23,10 @@ void mailbox_recent_flags_set_uid_forced(struct mailbox *box, uint32_t uid)
 {
 	box->recent_flags_prev_uid = uid;
 
-	seq_range_array_add_with_init(&box->recent_flags, 64, uid);
-	box->recent_flags_count++;
+	if (!mailbox_recent_flags_have_uid(box, uid)) {
+		seq_range_array_add_with_init(&box->recent_flags, 64, uid);
+		box->recent_flags_count++;
+	}
 }
 
 void mailbox_recent_flags_set_seqs(struct mailbox *box,