packetizer: av1: fix double free

If the p_extra is freed in the previous branch, it is also freed in this
branch without being reset to NULL before.

Regression from a95213e20d.

modules/packetizer/av1.c

@@ -159,6 +159,7 @@ static void UpdateDecoderFormat(decoder_t *p_dec)
     {
         free(p_dec->fmt_out.p_extra);
         p_dec->fmt_out.i_extra = 0;
+        p_dec->fmt_out.p_extra = NULL;
     }
 
     if(p_dec->fmt_out.i_extra <= 4)