fix splat without assignment; fix #2781

The parser generates NODE_NIL for tSTAR without argument in masgns. The
codegen didn't handle that.

src/codegen.c

@@ -964,6 +964,9 @@ gen_assignment(codegen_scope *s, node *tree, int sp, int val)
       genop_peep(s, MKOP_AB(OP_MOVE, cursp(), sp), val);
     }
     break;
+  /* splat without assignment */
+  case NODE_NIL:
+    break;
 
   default:
 #ifdef ENABLE_STDIO