imapc: Fix potential crash if initial sync fails.

Happened at least if mailbox had 0 mails and a failing mailbox_sync() was
called twice.

Fixes:
Panic: file imapc-sync.c: line 477 (imapc_sync_index): assertion failed: (mbox->sync_fetch_first_uid == 1)

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

@@ -483,7 +483,6 @@ static void imapc_sync_index(struct imapc_sync_context *ctx)
 		   adding new messages requires sync locking to avoid
 		   duplicates. */
 		imapc_sync_send_commands(ctx, mbox->sync_fetch_first_uid);
-		mbox->sync_fetch_first_uid = 0;
 	}
 
 	imapc_sync_expunge_finish(ctx);
@@ -500,6 +499,10 @@ static void imapc_sync_index(struct imapc_sync_context *ctx)
 	if (mbox->box.v.sync_notify != NULL)
 		mbox->box.v.sync_notify(&mbox->box, 0, 0);
 
+	if (!ctx->failed) {
+		/* reset only after a successful sync */
+		mbox->sync_fetch_first_uid = 0;
+	}
 	if (!mbox->initial_sync_done && !ctx->failed) {
 		imapc_initial_sync_check(ctx, FALSE);
 		mbox->initial_sync_done = TRUE;