plugins: imapsieve: Deal with messages being expunged concurrently by…

… the time Sieve filter is to be applied.

Before, this was an assertion that got triggered transiently during imaptest.

src/plugins/imapsieve/imap-sieve-storage.c

@@ -747,7 +747,10 @@ imap_sieve_mailbox_transaction_run(
 			continue;
 		}
 
-		i_assert(!mail->expunged);
+		if (mail->expunged) {
+			/* already gone for some reason */
+			continue;
+		}
 
 		/* Run scripts for this mail */
 		ret = imap_sieve_run_mail