Explicitly set encoder_context to nullptr in SimulcastEncoderAdapter after move.

Bug: None
Change-Id: Idac2efa73835ff5e1c37e1f19d7d2127c62e5900
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/361642
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42957}

media/engine/simulcast_encoder_adapter.cc

@@ -441,6 +441,7 @@ int SimulcastEncoderAdapter::InitEncode(
         parent, std::move(encoder_context),
         std::make_unique<FramerateController>(stream_codec.maxFramerate),
         stream_idx, stream_codec.width, stream_codec.height, is_paused);
+    encoder_context = nullptr;
   }
 
   // To save memory, don't store encoders that we don't use.