Bug in sieve script compilation

Hi, this is Rumata888 from hackerone. This is the other bug I found in sievec. If you try to compile the following script, it will result in a panic. The reason is: export expects its arguments to be either strings or lists of strings. When we issue an export command with a number immediately followed by an export command with a string argument, on parsing the second command sievec tries to merge the commands by merging the arguments. When it encounters an argument of unexpected type, it panics. Once again, I would advise unlinking invalid commands from ast, so that parsing of following commands was not dependent on previous irregularities.