journal-remote: fix hostname double-free on request_meta() error paths

journal-remote: fix hostname double-free on request_meta() error paths

request_handler() owns the hostname var and passes it by value to
request_meta(), which hands it to source_new(), which stores it in
source->importer.name without copying. If build_accept_encoding()
then fails, the hostname var is freed, and then the caller's
_cleanup_free_ frees it a second time.

Follow-up for 9ff48d0982fcb97923955685fe9fa4e0e67cb238