libavcodec/tiffenc: Use av_freep() avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/tiffenc.c

@@ -420,7 +420,7 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
         }
     }
     if (s->compr == TIFF_LZW)
-        av_free(s->lzws);
+        av_freep(&s->lzws);
     }
 
     s->num_entries = 0;