pp_sys.c: Remove redundant null checks

Nulls only get pushed on to the stack when pp_coreargs uses them to
represent missing optional arguments.  Ops that take * prototypes
will have had their arguments passed through rv2gv first, so they
should always be GVs.  GvIOn never returns null.  When given a GV
argument, it creates a new IO entry and returns that.  When given
any other argument it croaks.

Thank you to Daniel Dragan for providing a list of candidate pp
functions in <rt-4.0.18-29609-1387862001-129.120842-15-0@perl.org>
(ticket #120842).
op.c: newLOOPOP: Remove code supporting do-sub