picture_pool: fix double mutex_unlock if pic_lock fails

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

src/misc/picture_pool.c

@@ -253,7 +253,6 @@ picture_t *picture_pool_Get(picture_pool_t *pool)
             vlc_mutex_lock(&pool->lock);
             sys->in_use = false;
             pool->refs--;
-            vlc_mutex_unlock(&pool->lock);
             continue;
         }