MINOR: chunk: add a BUG_ON upon the next init_trash_buffer()

MINOR: chunk: add a BUG_ON upon the next init_trash_buffer()

The trash pool is initialized twice in haproxy, first during STG_POOL,
and 2nd after configuration parsing.

Doing alloc_trash_chunk() between this 2 phases can lead to strange
things if we are using it after, indeed the pool is destroyed and
trying to do a free_trash_chunk() or accessing the pointer will lead to
crashes.

This patch checks that we don't have used buffers from the trash pool
before initializing the pool again.