Fix use-after-move in //media/filters/fuchsia/

Fix use-after-move (potential) bugs found by the
"bugprone-use-after-move" clang-tidy check.

Bug: 1122844
Change-Id: Ie7e703e9270086bd5ef1e9c3a2911f8f76d28713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2563762
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Auto-Submit: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831979}

media/filters/fuchsia/fuchsia_video_decoder.cc

@@ -684,8 +684,9 @@ void FuchsiaVideoDecoder::SendInputPacket(
 
   DCHECK(in_flight_input_packets_.find(packet.buffer_index()) ==
          in_flight_input_packets_.end());
+  const size_t buffer_index = packet.buffer_index();
   in_flight_input_packets_.insert_or_assign(
-      packet.buffer_index(), InputDecoderPacket{std::move(packet)});
+      buffer_index, InputDecoderPacket{std::move(packet)});
 }
 
 void FuchsiaVideoDecoder::ProcessEndOfStream() {