imapc: Fix potential assert-crash when reconnecting during syncing

Fixes:
Panic: file imapc-sync.c: line 328 (imapc_initial_sync_check): assertion failed: (mail_index_is_expunged(view, lseq) || seq_range_exists(&ctx->mbox->delayed_expunged_uids, luid))

src/lib-storage/index/imapc/imapc-storage.c

@@ -628,6 +628,7 @@ static void imapc_mailbox_reopen(void *context)
 		mbox->sync_next_rseq = 1;
 	}
 
+	mbox->state_fetched_success = FALSE;
 	mbox->initial_sync_done = FALSE;
 	mbox->selecting = TRUE;
 	mbox->selected = FALSE;

src/lib-storage/index/imapc/imapc-sync.c

@@ -411,6 +411,12 @@ static void imapc_sync_index(struct imapc_sync_context *ctx)
 		imapc_mailbox_run(mbox);
 	array_free(&ctx->expunged_uids);
 
+	if (!mbox->state_fetched_success) {
+		/* All the sync commands succeeded, but we got disconnected.
+		   imapc_initial_sync_check() will crash if we go there. */
+		ctx->failed = TRUE;
+	}
+
 	/* add uidnext & highestmodseq after all appends */
 	imapc_sync_uid_next(ctx);
 	imapc_sync_highestmodseq(ctx);