virtual: If virtual_uid is not assigned skip it

We cannot lookup mails with uid 0, so if we see
that mail has virtual uid 0, do not attempt to look it
up from index, but skip it.

Fixes Panic: file mail-index-map.c: line 549 (mail_index_map_lookup_seq_range): assertion failed: (first_uid > 0)

src/plugins/virtual/virtual-sync.c

@@ -567,6 +567,10 @@ virtual_sync_mailbox_box_remove(struct virtual_sync_context *ctx,
 		for (; uid <= uids[i].seq2; uid++, src++) {
 			i_assert(src < rec_count);
 			i_assert(uidmap[src].real_uid == uid);
+			if (uidmap[src].virtual_uid == 0) {
+				/* has not been assigned yet */
+				continue;
+			}
 			if (mail_index_lookup_seq(ctx->sync_view,
 						  uidmap[src].virtual_uid,
 						  &vseq))
@@ -1040,6 +1044,10 @@ static int virtual_sync_backend_box_sync(struct virtual_sync_context *ctx,
 			uidmap = array_idx(&bbox->uids, 0);
 			for (; idx1 <= idx2; idx1++) {
 				vuid = uidmap[idx1].virtual_uid;
+				if (vuid == 0) {
+					/* has not been even assigned yet */
+					continue;
+				}
 				if (!mail_index_lookup_seq(ctx->sync_view,
 							   vuid, &vseq)) {
 					/* expunged by another session,