plugins: imap-filter-sieve: Add assertion on attempting to execute at…

… least one script.

Coverity complains about last_script possibly being empty in
imap_sieve_filter_run_scripts(), which is actually not possible, since the
function would not be called if there is no script to execute. Added assertion
to make that clear.

src/plugins/imap-filter-sieve/imap-filter-sieve.c

@@ -773,6 +773,7 @@ imap_sieve_filter_run_scripts(struct imap_filter_sieve_context *sctx,
 		return 1;
 	}
 
+	i_assert(last_script != NULL); /* at least one script is executed */
 	return imap_sieve_filter_handle_exec_status(sctx,
 		last_script, ret, keep, scriptenv->exec_status);
 }