Don't crash on FreeBuffer() with zero-length slice.

fuse/bufferpool.go

@@ -106,7 +106,7 @@ func (me *BufferPool) FreeBuffer(slice []byte) {
 	if cap(slice) < PAGESIZE {
 		return
 	}
-
+	slice = slice[:cap(slice)] 
 	key := uintptr(unsafe.Pointer(&slice[0]))
 	exp, ok := me.outstandingBuffers[key]
 	if ok {

fuse/bufferpool_test.go

@@ -39,3 +39,9 @@ func TestBufferPool(t *testing.T) {
 	}
 
 }
+
+func TestFreeBufferEmpty(t *testing.T) {
+	bp := NewBufferPool()
+	c := make([]byte, 0, 2*PAGESIZE)
+	bp.FreeBuffer(c)
+}