pp_leaveeval: reset stack in VOID context

pp_leaveeval: reset stack in VOID context

    $ perl -Dst -e'eval"1"'

Gives:
    ....
    ((eval 1):1)	leaveeval
        =>  (FREED)

Change it so that like all the other pp_leavefoo() functions, it does

    if (gimme == G_VOID)
        PL_stack_sp = newsp;

I can't think of any (non-debugging) perl-level badness the old behaviour
can be shown to demonstrate, but best not to have freed values left
dangling.

This also allows pp_leaveeval() to (like the other pp_leavefoo functions)
avoid doing a dSP with the associated unnecessary PUTBACKs and SPAGAINs.

Finally, the code to detect a false require is moved to earlier in the
function where it's in the same place as the rest of the stack arg
processing code.