gh-1649: regression caused by the partial fix for gh-846

* the bug with iobuf recycling has always been in 1.6,
  and is fixed in 1.7, but got more exposure with the patch
  for gh-846. "Idle" state of iobuf was calculated incorrectly.

src/iobuf.h

@@ -90,7 +90,7 @@ iobuf_reset_mt(struct iobuf *iobuf);
 static inline bool
 iobuf_is_idle(struct iobuf *iobuf)
 {
-	return ibuf_used(&iobuf->in) == 0 && obuf_size(&iobuf->out) == 0;
+	return ibuf_used(&iobuf->in) == 0 && obuf_used(&iobuf->out) == 0;
 }
 
 /**