Bug in sieve script compilation

[Иннокентий Сенновский]

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.
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