Be more cautious when releasing RTCRtpSender's encoder (fixes: aiortc#1124)

We used `del` to forcibly drop the reference to the encoder and allow it
to be garbage collected. However it also means that the `RTCRtpSender`
no longer has an `__encoder` attribute from then on. We can encounter a
race condition during shutdown where we receive an RTCP packet, which
causes access to the removed `__encoder` attribute.

Instead, just set the `__encoder` attribute back to `None`, as when the
`RTCPRtpSender` was constructed.

Author: jlaine

src/aiortc/rtcrtpsender.py

@@ -406,8 +406,7 @@ async def _run_rtp(self, codec: RTCRtpCodecParameters) -> None:
             self.__track = None
 
         # release encoder
-        if self.__encoder:
-            del self.__encoder
+        self.__encoder = None
 
         self.__log_debug("- RTP finished")
         self.__rtp_exited.set()