pp.c:pp_coreargs: use PL_op_desc instead of OP_DESC

pp.c:pp_coreargs: use PL_op_desc instead of OP_DESC

Instead of using OP_DESC on op_next, since we already know the op
number, we can just go straight to PL_op_desc, which is what OP_DESC
itself uses after looking inside op_next to find out what the op
number is.

BTW, &CORE::scalar will need this, since a scalar op is never actu-
ally executed, making coreargs’ op_next pointer point to another op
than expected:

2  <1> leavesublv[1 ref] K/REFC,1 ->(end)
-     <1> scalar sKP/1 ->2
1        <$> coreargs(IV 2) s ->2
-e syntax OK

Otherwise we get ‘Not enough arguments for lvalue subroutine return’.