Description:
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imap_sort() accepts Search_criteria argument. When the value is unparsable, c-client  
returns error (it is accessible via imap_last_error()). In this case imap_sort() 
should to return false to flag the problem. 

Test script:
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Run imap_sort() with any invalid search_criteria

Expected result:
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False should be returned

Actual result:
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Empty array is returned