Perl_scalar: explicitly handle OP_ASLICE case

`Perl_scalar` does not explicitly handle the OP_ASLICE case and
consequently some OPs are not nulled, even though the user-visible
behaviour would suggest that has happened.

For example, `my $y = @x[4,3,2];` would give the optree:
```
b     <;> nextstate(main 9 -e:1) v:%,us,{,fea=15 ->c
i     <1> padsv_store[$y:9,10] vKS/LVINTRO ->j
h        <@> aslice sK ->i
c           <0> pushmark s ->d
-           <1> ex-list lK ->g
-              <0> ex-pushmark s ->d
d              <$> const(IV 4) s ->e
e              <$> const(IV 3) s ->f
f              <$> const(IV 2) s ->g
g           <0> padav[@x:8,10] sR ->h
```

After this commit:
```
b     <;> nextstate(main 9 -e:1) v:%,us,{,fea=15 ->c
g     <1> padsv_store[$y:9,10] vKS/LVINTRO ->h
f        <@> aslice sK ->g
c           <0> pushmark s ->d
-           <1> ex-list lK ->e
-              <0> ex-pushmark v ->-
-              <0> ex-const v ->-
-              <0> ex-const v ->d
d              <$> const(IV 2) s ->e
e           <0> padav[@x:8,10] sR ->f
```

The user-visible behaviour should be unchanged, but now only one
element of @x will be removed, rather than three with two of them
being immediately discarded.

Author: richardleach

op.c

@@ -2027,6 +2027,19 @@ Perl_scalar(pTHX_ OP *o)
         case OP_LIST:
             kid = cLISTOPo->op_first;
             goto do_kids;
+        case OP_ASLICE: {
+                OP * first = cLISTOPo->op_first;
+                assert(OP_TYPE_IS(first, OP_PUSHMARK));
+                assert(first->op_moresib);
+                OP * sib = OpSIBLING(first);
+                if(OP_TYPE_IS_OR_WAS(sib, OP_LIST)) {
+                    kid = cLISTOPx(sib)->op_first;
+                    goto do_kids;
+                }
+                if (o->op_flags & OPf_KIDS)
+                    next_kid = cUNOPo->op_first; /* do all kids */
+                break;
+            }
         case OP_LSLICE:
                 assert(OP_TYPE_IS_OR_WAS(cLISTOPo->op_first, OP_LIST));
                 kid = cLISTOPx(cLISTOPo->op_first)->op_first;