lib-sieve: Fix assert panic occurring when used command has external …

…tag, but is not registered.

This occurred for example when the "fileinto" command is used with only the
"copy" extension in the require line.

Panic message was:

Panic: file hash.c: line 263 (hash_table_insert_node): assertion failed: (opcode == HASH_TABLE_OP_UPDATE)

src/lib-sieve/sieve-validator.c

@@ -352,8 +352,16 @@ void sieve_validator_register_command
 static void sieve_validator_register_unknown_command
 (struct sieve_validator *valdtr, const char *command)
 {
-	(void)_sieve_validator_register_command
-		(valdtr, NULL, &unknown_command, command);
+	struct sieve_command_registration *cmd_reg =
+		sieve_validator_find_command_registration(valdtr, command);
+
+	if (cmd_reg == NULL) {
+		(void)_sieve_validator_register_command
+			(valdtr, NULL, &unknown_command, command);
+	} else {
+		i_assert(cmd_reg->cmd_def == NULL);
+		cmd_reg->cmd_def = &unknown_command;
+	}
 }
 
 /*const struct sieve_command *sieve_validator_find_command