log: fix use-after-free

Introduced in commit cb7e0ace739c.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/backend.c b/backend.c
index 95a7572..448fc59 100644
--- a/backend.c
+++ b/backend.c
@@ -2047,6 +2047,7 @@
 				struct io_log *log = agg_io_log[i];
 
 				__finish_log(log);
+				free_log(log);
 			}
 		}
 	}