revert r40760

* vm.c (vm_invoke_proc): revert r40760 partially, which causes timeout
  in test/ruby/test_thread.rb:test_stack_size.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40767 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm.c

@@ -682,7 +682,7 @@ static VALUE
 vm_invoke_proc(rb_thread_t *th, rb_proc_t *proc, VALUE self, VALUE defined_class,
 	       int argc, const VALUE *argv, const rb_block_t *blockptr)
 {
-    volatile VALUE val = Qundef;
+    VALUE val = Qundef;
     int state;
     volatile int stored_safe = th->safe_level;