MultiTask has no input tasks

Scenario:

1. Open the WD.
2. Add "Read sequence" element and set "human_T1.fa" file from UGENE sample data as input.
3. Add "In Silico PCR" element, set the attached "long_primers.fa" file as "Primers URL".
4. Connect elements and click "Run".
   Expected result: the workflow finishes with an error, because all primers are too long (the limit for each primer is 1024 characters).
   Actual result: a SAFE_POINT is triggered.

The technical reason: the constructor of MultiTask class has several SAFE_POINTs. In case one of them is triggered, a member of the class is left not initialized. It's necessary to reorganize the constructor to avoid such behaviour.

See also crash #22665.