lavc/videotoolboxenc: make transfer_fnc initialized for unsupport function

The current function will report one error message, but the caller func
haven't check it, so change the default to process as AVCOL_TRC_UNSPECIFIED.
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Rick Kern <kernrj@gmail.com>

libavcodec/videotoolboxenc.c

@@ -949,6 +949,7 @@ static int get_cv_transfer_function(AVCodecContext *avctx,
             break;
 
         default:
+            *transfer_fnc = NULL;
             av_log(avctx, AV_LOG_ERROR, "Transfer function %s is not supported.\n", av_color_transfer_name(trc));
             return -1;
     }