pp.c:pp_gelem: Less stack fiddling

Instead of pop, pop, push, just do one pop and then replace the
remaining item.  Also, there is no need to extend the stack here, as
pp_gelem returns fewer items than it consumes.
pp.c:pp_study: Less stack fiddling

We always pop and push exactly one item.  We can skip the sp++ and
sp--, and also the PUTBACK.