[perl #108276] avoid deep scalarvoid() self-recursion

This fixes the original test case from the ticket.

It may be possible for code that alternates recursion between
scalarvoid() and scalar() to cause deep recursion, but I haven't seen
an example that does it.

This doesn't indent the scalarvoid() loop, which will be done in another
change.
move the closing ) out of the #ifdef USE_ITHREADS to avoid confusing tools

emacs, in my case
[perl #108276] re-indent the loop body