Fix with_acquire: release a non locked mutex is UB

This is segfaulting (on ubuntu 16.04 and 16.10) when trying to release the unlocked mutex in the release stage (in general this is an undefined behavior).

src/threads/CCSemaphore.ml

@@ -53,14 +53,13 @@ let release m t =
 *)
 
 let with_acquire ~n t ~f =
-  Mutex.lock t.mutex;
-  acquire_once_locked_ n t;
+  acquire n t;
   try
     let x = f() in
-    release_once_locked_ n t;
+    release n t;
     x
   with e ->
-    release_once_locked_ n t;
+    release n t;
     raise e
 
 (*$R