gc.c: never call dmark for NULL

* gc.c (gc_mark_children): call dmark function for non-NULL
  pointers only, so that DATA_PTR can be NULL safely now.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50462 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Sun May 10 21:32:45 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* gc.c (gc_mark_children): call dmark function for non-NULL
+	  pointers only, so that DATA_PTR can be NULL safely now.
+
 Sun May 10 16:23:58 2015  Kazuki Tsujimoto  <kazuki@callcc.net>
 
 	* proc.c (proc_binding): fix segmentation fault on marking phase.

gc.c

@@ -4300,12 +4300,14 @@ gc_mark_children(rb_objspace_t *objspace, VALUE obj)
 	break;
 
       case T_DATA:
-	if (RTYPEDDATA_P(obj)) {
-	    RUBY_DATA_FUNC mark_func = any->as.typeddata.type->function.dmark;
-	    if (mark_func) (*mark_func)(DATA_PTR(obj));
-	}
-	else {
-	    if (any->as.data.dmark) (*any->as.data.dmark)(DATA_PTR(obj));
+	{
+	    void *const ptr = DATA_PTR(obj);
+	    if (ptr) {
+		RUBY_DATA_FUNC mark_func = RTYPEDDATA_P(obj) ?
+		    any->as.typeddata.type->function.dmark :
+		    any->as.data.dmark;
+		if (mark_func) (*mark_func)(ptr);
+	    }
 	}
 	break;