Set length in INVOKEDYNAMIC constructor

[rootvector2]

found while checking instruction getLength() against dump(): the INVOKEDYNAMIC(int) constructor never sets its length, so a generated invokedynamic reports 3 while dump() writes 5 bytes and InstructionList.setPositions() then places every following instruction 2 bytes too low; mirror the sibling INVOKEINTERFACE/MULTIANEWARRAY constructors (and this class's own initFromFile) by calling setLength(5).

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