Its documentation states that it is allocated/freed by the caller, but
it is declared as an AV_OPT_TYPE_STRING AVOption. Since
36773283 the AVOptions system frees
strings automatically. This can be considered an API break, since it
won't work when the caller doesn't use av_malloc() to allocate the
memory or wants to use the string after closing the codec.

Since there is not much value in this field being an AVOption, the best
solution is to remove it from the options table.