handle_range_longs(): refcount handling is very delicate here, and

handle_range_longs():  refcount handling is very delicate here, and
the code erroneously decrefed the istep argument in an error case.  This
caused a co_consts tuple to lose a float constant prematurely, which
eventually caused gc to try executing static data in floatobject.c (don't
ask <wink>).  So reworked this extensively to ensure refcount correctness.