pp_aassign(): fix ($x,$y) = (undef, $x)

pp_aassign(): fix ($x,$y) = (undef, $x)

With 808ce5578203, I tweaked the OPpASSIGN_COMMON flagging to mark as safe
when the LHS or RHS only contains only one var. This turned out to be
flawed for the RHS logic, as position as well as oneness is important:

   (undef, $x) = ...;         # only 1 scalar on LHS: always safe
   ($x, $y)    = (undef, $x); # 2 scalars on RHS: unsafe

So this commit makes undef on the RHS count towards the scalar var count.