temporary add some extra renderbuffer debug code
diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index c24e2e3..58efa0b 100644
--- a/src/mesa/main/mtypes.h
+++ b/src/mesa/main/mtypes.h
@@ -2222,6 +2222,8 @@
  */
 struct gl_renderbuffer
 {
+#define RB_MAGIC 0xaabbccdd
+   int Magic; /** XXX TEMPORARY DEBUG INFO */
    _glthread_Mutex Mutex;		   /**< for thread safety */
    GLuint ClassID;        /**< Useful for drivers */
    GLuint Name;
diff --git a/src/mesa/main/renderbuffer.c b/src/mesa/main/renderbuffer.c
index 1145e91..6f1d7c3 100644
--- a/src/mesa/main/renderbuffer.c
+++ b/src/mesa/main/renderbuffer.c
@@ -1471,6 +1471,7 @@
 {
    _glthread_INIT_MUTEX(rb->Mutex);
 
+   rb->Magic = RB_MAGIC;
    rb->ClassID = 0;
    rb->Name = name;
    rb->RefCount = 0;
@@ -2149,21 +2150,26 @@
       GLboolean deleteFlag = GL_FALSE;
       struct gl_renderbuffer *oldRb = *ptr;
 
+      assert(oldRb->Magic == RB_MAGIC);
       _glthread_LOCK_MUTEX(oldRb->Mutex);
+      assert(oldRb->Magic == RB_MAGIC);
       ASSERT(oldRb->RefCount > 0);
       oldRb->RefCount--;
       /*printf("RB DECR %p (%d) to %d\n", (void*) oldRb, oldRb->Name, oldRb->RefCount);*/
       deleteFlag = (oldRb->RefCount == 0);
       _glthread_UNLOCK_MUTEX(oldRb->Mutex);
 
-      if (deleteFlag)
+      if (deleteFlag) {
+         oldRb->Magic = 0; /* now invalid memory! */
          oldRb->Delete(oldRb);
+      }
 
       *ptr = NULL;
    }
    assert(!*ptr);
 
    if (rb) {
+      assert(rb->Magic == RB_MAGIC);
       /* reference new renderbuffer */
       _glthread_LOCK_MUTEX(rb->Mutex);
       rb->RefCount++;