avcodec/mfenc: check IMFSample_ConvertToContiguousBuffer() for failure

Fixes: CID1591911 Logically dead code

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 86cd7c68bc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mfenc.c

@@ -248,7 +248,7 @@ static int mf_sample_to_avpacket(AVCodecContext *avctx, IMFSample *sample, AVPac
     if ((ret = ff_get_encode_buffer(avctx, avpkt, len, 0)) < 0)
         return ret;
 
-    IMFSample_ConvertToContiguousBuffer(sample, &buffer);
+    hr = IMFSample_ConvertToContiguousBuffer(sample, &buffer);
     if (FAILED(hr))
         return AVERROR_EXTERNAL;