vm_core.h: sandwich jmpbuf

* vm_core.h (rb_vm_tag): move jmpbuf which should be stored at the
  last in TH_PUSH_TAG, between tag and state so ensure to be
  accessible.  reapplied r40806.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59160 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_core.h

@@ -691,11 +691,16 @@ typedef RUBY_JMP_BUF rb_jmpbuf_t;
 struct rb_vm_tag {
     VALUE tag;
     VALUE retval;
+    rb_jmpbuf_t buf;
     struct rb_vm_tag *prev;
     enum ruby_tag_type state;
-    rb_jmpbuf_t buf;
 };
 
+STATIC_ASSERT(rb_vm_tag_buf_offset, offsetof(struct rb_vm_tag, buf) > 0);
+STATIC_ASSERT(rb_vm_tag_buf_end,
+	      offsetof(struct rb_vm_tag, buf) + sizeof(rb_jmpbuf_t) <
+	      sizeof(struct rb_vm_tag));
+
 struct rb_vm_protect_tag {
     struct rb_vm_protect_tag *prev;
 };