BUG/MINOR: ring: release the backing store name on exit

ASAN found that a ring equipped with a backing store did not release
the store name on exit.

This should be backported to 2.7.

src/sink.c

@@ -1398,6 +1398,7 @@ static void sink_deinit()
 
 				msync(area, size, MS_SYNC);
 				munmap(area, size);
+				ha_free(&sink->store);
 			}
 			else
 				ring_free(sink->ctx.ring);