BUG/MINOR: ssl/ckch: free the new instance when ckch_inst_rebuild() fails

BUG/MINOR: ssl/ckch: free the new instance when ckch_inst_rebuild() fails

ckch_inst_rebuild() creates the new ckch_inst of a "commit ssl cert" with
its sni_ctx and SSL_CTX, then prepares the SSL_CTX of the server or of
the bind line. When this preparation fails it returns an error to
ckch_store_rebuild_instances() which only returns -1, so the new instance,
which is not linked anywhere yet, is leaked with its SSL_CTXs, and with
everything they reference (the certificate, the chain, the key, the CA
file links).

The impact is a memory leak for each failed commit, which requires an
admin to commit a certificate that doesn't fit its bind or server lines.

Free the instance with ckch_inst_free() and reset the pointer on these
two error paths. The instance is not in any tree nor list at this point,
which ckch_inst_free() handles. The first error path, after
ckch_inst_new_load_store() or ckch_inst_new_load_srv_store(), is already
fine since these functions never leave an instance behind on failure.

This should be backported to all stable branches, the function exists
since 2.5.