parse.y: fix block passing with empty kwargs

* parse.y (arg_blk_pass): preceeding arguments node may be NULL when
  an empty keyword argument hash splat is optimized away.
  [ruby-core:88890] [Bug #15087]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64786 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

parse.y

@@ -9962,6 +9962,7 @@ static NODE *
 arg_blk_pass(NODE *node1, NODE *node2)
 {
     if (node2) {
+        if (!node1) return node2;
 	node2->nd_head = node1;
 	nd_set_first_lineno(node2, nd_first_lineno(node1));
 	nd_set_first_column(node2, nd_first_column(node1));

test/ruby/test_keyword.rb

@@ -563,7 +563,8 @@ class TestKeywordArguments < Test::Unit::TestCase
 
   def test_dynamic_symbol_keyword
     bug10266 = '[ruby-dev:48564] [Bug #10266]'
-    assert_separately(['-', bug10266], <<-'end;') #    do
+    assert_separately(['-', bug10266], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
       bug = ARGV.shift
       "hoge".to_sym
       assert_nothing_raised(bug) {eval("def a(hoge:); end")}
@@ -746,4 +747,8 @@ class TestKeywordArguments < Test::Unit::TestCase
 
     assert_equal(:ok, many_kwargs(e0: :ok)[i], "#{i}: e0"); i+=1
   end
+
+  def test_splat_empty_hash_with_block_passing
+    assert_valid_syntax("bug15087(**{}, &nil)")
+  end
 end