DmaBufAllocator: Avoid syncing with an invalid file descriptor

As DmaSyncer disables the copy c'tor, the move c'tor will be used
instead. This leaves some DmaSyncers with invalid SharedFDs. They should
avoid syncing with invalid file descriptors in the d'tor.

Fixes: 545046a4 ("DmaBufAllocator: Make DmaSyncer non-copyable")
Signed-off-by: Harvey Yang <chenghaoyang@chromium.org>

src/libcamera/dma_buf_allocator.cpp

@@ -330,6 +330,11 @@ DmaSyncer::~DmaSyncer()
 
 void DmaSyncer::sync(uint64_t step)
 {
+	// DmaSyncer might be moved and left an empty SharedFD.
+	// Avoid syncing with an invalid file descriptor in this case.
+	if (!fd_.isValid())
+		return;
+
 	struct dma_buf_sync sync = {
 		.flags = flags_ | step
 	};