lavc/videotoolboxenc: Fix crash when closing codec after error

Fixes crash in #5352. VTCompressionSessionInvalidate() crashes if the
internal encoder hasn't completed, but hasn't experienced an error.
The function call isn't needed since the encoder is invalidated when
the reference count reaches 0 anyway.

Signed-off-by: Rick Kern <kernrj@gmail.com>
Signed-off-by: wm4 <nfxjfg@googlemail.com>

libavcodec/videotoolboxenc.c

@@ -1275,7 +1275,6 @@ static av_cold int vtenc_close(AVCodecContext *avctx)
 
     if(!vtctx->session) return 0;
 
-    VTCompressionSessionInvalidate(vtctx->session);
     pthread_cond_destroy(&vtctx->cv_sample_sent);
     pthread_mutex_destroy(&vtctx->lock);
     CFRelease(vtctx->session);