Fix a bug related to moving signing_key_cert

Now that the field exists in signed_descriptor_t, we need to make
sure we free it when we free a signed_descriptor_t, and we need to
make sure that we don't free it when we convert a routerinfo_t to a
signed_descriptor_t.

But not in any released Tor. I found this while working on #19128.

One problem: I don't see how this could cause 19128.