Fix nested empty heredoc causing segfault

As reported by https://hackerone.com/jpenalbae

mrbgems/mruby-compiler/core/codegen.c

@@ -2285,7 +2285,11 @@ codegen(codegen_scope *s, node *tree, int val)
     if (val) {
       node *n = tree;
 
-      if (!n) break;
+      if (!n) {
+        genop(s, MKOP_A(OP_LOADNIL, cursp()));
+        push();
+        break;
+      }
       codegen(s, n->car, VAL);
       n = n->cdr;
       while (n) {

mrbgems/mruby-compiler/core/parse.y

@@ -6541,7 +6541,7 @@ mrb_parser_dump(mrb_state *mrb, node *tree, int offset)
 
   case NODE_HEREDOC:
     printf("NODE_HEREDOC (<<%s):\n", ((parser_heredoc_info*)tree)->term);
-    mrb_parser_dump(mrb, ((parser_heredoc_info*)tree)->doc, offset+1);
+    dump_recur(mrb, ((parser_heredoc_info*)tree)->doc, offset+1);
     break;
 
   default:

test/t/codegen.rb

+##
+# Codegen tests
+
+assert('nested empty heredoc') do
+  _, a = nil, <<B
+#{<<A}
+A
+B
+  assert_equal "\n", a
+end