Fix crash in Dir#each/each_child/scan when closed

If the Dir object is closed in the block of Dir#each/each_child/scan, then
it will crash because dirp->dir will be a NULL pointer. The following
script demonstrates the crash:

    d = Dir.open("/")
    d.each { d.close }

Author: peterzhu2118

dir.c

@@ -1102,6 +1102,8 @@ dir_each_entry(VALUE dir, VALUE (*each)(VALUE, VALUE, struct dir_entry_args *),
             .dp = dp,
         };
         (*each)(arg, path, &each_args);
+        /* the block may have closed dir */
+        if (!dirp->dir) dir_closed();
     }
     return dir;
 }

test/ruby/test_dir.rb

@@ -237,6 +237,17 @@ def test_close
     assert_raise(IOError) { d.read }
   end
 
+  def test_each_with_close
+    d = Dir.open(@root)
+    assert_raise(IOError) { d.each { d.close } }
+
+    d = Dir.open(@root)
+    assert_raise(IOError) { d.each_child { d.close } }
+
+    d = Dir.open(@root)
+    assert_raise(IOError) { d.scan { |*| d.close } }
+  end
+
   def test_glob
     assert_equal((%w(.) + ("a".."z").to_a).map{|f| File.join(@root, f) },
                  Dir.glob(File.join(@root, "*"), File::FNM_DOTMATCH))