avutil/buffer: use appropriate atomic operations

No functional changes. ref/unref vs add/sub is symmetrical.

Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/buffer.c

@@ -116,7 +116,7 @@ static void buffer_replace(AVBufferRef **dst, AVBufferRef **src)
     } else
         av_freep(dst);
 
-    if (atomic_fetch_add_explicit(&b->refcount, -1, memory_order_acq_rel) == 1) {
+    if (atomic_fetch_sub_explicit(&b->refcount, 1, memory_order_acq_rel) == 1) {
         b->free(b->opaque, b->data);
         av_freep(&b);
     }
@@ -281,7 +281,7 @@ void av_buffer_pool_uninit(AVBufferPool **ppool)
     pool   = *ppool;
     *ppool = NULL;
 
-    if (atomic_fetch_add_explicit(&pool->refcount, -1, memory_order_acq_rel) == 1)
+    if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)
         buffer_pool_free(pool);
 }
 
@@ -298,7 +298,7 @@ static void pool_release_buffer(void *opaque, uint8_t *data)
     pool->pool = buf;
     ff_mutex_unlock(&pool->mutex);
 
-    if (atomic_fetch_add_explicit(&pool->refcount, -1, memory_order_acq_rel) == 1)
+    if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)
         buffer_pool_free(pool);
 }