LDA Sieve plugin: Fixed sequential exection of LDAP-based scripts.

The sequence was broken when the LDAP script attribute does not exist; i.e., when the script is not found.
The NOT_FOUND error is returned at a later instance than normal fs-based scripts, explaining why this problem does not normally occur.

src/plugins/lda-sieve/lda-sieve-plugin.c

@@ -554,6 +554,10 @@ static int lda_sieve_execute_scripts
 
 		more = lda_sieve_execute_script(srctx, mscript,
 			script, i, discard_script, &error);
+		if ( error == SIEVE_ERROR_NOT_FOUND ) {
+			/* skip scripts which finally turn out not to exist */
+			more = TRUE;
+		}
 
 		if ( discard_script ) {
 			/* Executed discard script, which is always final */