http: make evhttp_connection_get_addr() return const
regress_http: add const to get_addr::storage
http: we don't need to NULL conn_addr when we freeing old connection

Because after it the whole struct ehttp_connection will be free'd.
http: freeing "conn_address" only if we really have it.

As Nick pointed:
https://github.com/azat/libevent/commit/a7a14f0b65bcdb7f89a6418625b5fac8f7de3a0e#commitcomment-4302501