vm: produce better error for ROLL with wrong index

Current VM implementation doesn't return errors for many operations, so the
only way to handle it here is to check for NULL. Refs. #96.

pkg/vm/vm.go

@@ -342,7 +342,11 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 			panic("negative stack item returned")
 		}
 		if n > 0 {
-			v.estack.Push(v.estack.RemoveAt(n))
+			e := v.estack.RemoveAt(n)
+			if e == nil {
+				panic("bad index")
+			}
+			v.estack.Push(e)
 		}
 
 	case DROP: