BUG/MINOR: build: Fix compilation with debug mode enabled

It remained some fragments of the old buffers API in debug messages, here and
there.

This was caused by the recent buffer API changes, no backport is needed.
diff --git a/src/backend.c b/src/backend.c
index e94c4c9..b2b7ba5 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1399,13 +1399,13 @@
 	uint32_t addr;
 	char *p;
 
-	DPRINTF(stderr,"[%u] %s: stream=%p b=%p, exp(r,w)=%u,%u bf=%08x bh=%d analysers=%02x\n",
+	DPRINTF(stderr,"[%u] %s: stream=%p b=%p, exp(r,w)=%u,%u bf=%08x bh=%lu analysers=%02x\n",
 		now_ms, __FUNCTION__,
 		s,
 		req,
 		req->rex, req->wex,
 		req->flags,
-		req->buf->i,
+		ci_data(req),
 		req->analysers);
 
 	if (s->flags & SF_ASSIGNED)