handleSetMember crashed while decompiling AS2 code that applies `--` to a
virtual (`__get__`/`__set__`) property, e.g. `this.prop--` where `prop` is
defined via addProperty. The decrement-setter branch built a
PreDecrementActionItem but cast the value to IncrementActionItem instead of
DecrementActionItem, throwing ClassCastException.
That exception was then masked: it propagated into the finally block before
addToOutput ran, so `output` was empty and `output.remove(output.size() - 1)`
threw IndexOutOfBoundsException, replacing the real cause. The reported error
was therefore always a misleading "Index -1 out of bounds for length 0",
which also aborts loading entirely when "automatically rename identifiers" is
enabled (deobfuscateIdentifiers decompiles every script on load).
Fixes:
- Cast value to DecrementActionItem in the PreDecrement setter branch.
- Guard the finally against an empty output (mirrors cleanupTemp) so a future
exception in the try block is no longer masked.
Affected code now decompiles correctly to `++prop` / `--prop`.
