ffmpeg: leak when re-allocating ffmpeg context

The _Setup method can be called several times if the parameters are
changed (for example when calling SetQuality). But it always allocated a
new context without freeinthe previous one, resulting in a leak.

Part of #9458, it fixes the largest leak there (10K bytes for each loop
of the provided test) but there are other leaks left as well, some of
which apparently need an update to a newer ffmpeg version removing some
deprecated fields.

Change-Id: Iad119dbda9cff1ce023b602a1978b100a1263f7d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5953
Reviewed-by: waddlesplash <[email protected]>

src/add-ons/media/plugins/ffmpeg/AVCodecEncoder.cpp

@@ -271,6 +271,11 @@ AVCodecEncoder::_Setup()
 
 	int rawBitRate;
 
+	if (fCodecContext != NULL) {
+		avcodec_close(fCodecContext);
+		avcodec_free_context(&fCodecContext);
+	}
+
 	fCodecContext = avcodec_alloc_context3(fCodec);
 	if (fCodecContext == NULL)
 		return B_NO_INIT;