gc.h: evaluate the macro argument only once

* gc.h (RUBY_MARK_UNLESS_NULL): evaluate the argument only once
  to get rid of inadvertent side effects.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51247 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Wed Jul 15 08:59:19 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* gc.h (RUBY_MARK_UNLESS_NULL): evaluate the argument only once
+	  to get rid of inadvertent side effects.
+
 Wed Jul 15 02:53:11 2015  Koichi Sasada  <ko1@atdot.net>
 
 	* vm_core.h, vm.c: remove rb_proc_t::envval because we can know it via

gc.h

@@ -57,7 +57,10 @@ rb_gc_debug_body(const char *mode, const char *msg, int st, void *ptr)
 #define RUBY_GC_INFO if(0)printf
 #endif
 
-#define RUBY_MARK_UNLESS_NULL(ptr) if(RTEST(ptr)){rb_gc_mark(ptr);}
+#define RUBY_MARK_UNLESS_NULL(ptr) do { \
+    VALUE markobj = (ptr); \
+    if (RTEST(markobj)) {rb_gc_mark(markobj);} \
+} while (0)
 #define RUBY_FREE_UNLESS_NULL(ptr) if(ptr){ruby_xfree(ptr);(ptr)=NULL;}
 
 #if STACK_GROW_DIRECTION > 0