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

test/t/syntax.rb

@@ -213,6 +213,12 @@ assert('Splat and multiple assignment in for') do
   assert_equal 7, f
 end
 
+assert('Splat without assignment') do
+  * = [0]
+  a, * = [1, 2]
+  assert_equal 1, a
+end
+
 assert('Return values of case statements') do
   a = [] << case 1
   when 3 then 2