Fix a couple of uninitialised variables.

throttled_by_backlog is quite new, so it's not too surprising that I
hadn't already noticed it wasn't initialised. But the failure to null
out the final 'next' pointer in the callbacks list when it's rewritten
as a result of delete_callbacks_for_context is a great deal older, so
I'm a lot more puzzled about how it hasn't come up until now!

callback.c

@@ -65,6 +65,7 @@ void delete_callbacks_for_context(void *ctx)
 
     cbhead = newhead;
     cbtail = newtail;
+    newtail->next = NULL;
 }
 
 void queue_toplevel_callback(toplevel_callback_fn_t fn, void *ctx)