assert that no hook_entry is registered twice

ir/ir/irdump.c

@@ -1124,7 +1124,7 @@ static void dump_node_vcgattr(FILE *F, ir_node *node, ir_node *local, int bad)
 
 void *dump_add_node_info_callback(dump_node_info_cb_t *cb, void *data)
 {
-	hook_entry_t *info = XMALLOC(hook_entry_t);
+	hook_entry_t *info = XMALLOCZ(hook_entry_t);
 
 	info->hook._hook_node_info = cb;
 	info->context              = data;

ir/ir/irhooks.c

@@ -25,6 +25,8 @@
  */
 #include "config.h"
 
+#include <assert.h>
+
 #include "irhooks.h"
 
 /* the hooks */
@@ -37,6 +39,9 @@ void register_hook(hook_type_t hook, hook_entry_t *entry)
   if (! entry->hook._hook_turn_into_id)
     return;
 
+  /* hook should not be registered yet */
+  assert(entry->next == NULL && hooks[hook] != entry);
+
   entry->next = hooks[hook];
   hooks[hook] = entry;
 }