http: freeing conn_address in evhttp_connection_reset_()

This is more proper, also without this patch valgrind complain about
losted bytes.

==22060== 128 bytes in 1 blocks are definitely lost in loss record 2 of 2
==22060==    at 0x4C2935B: malloc (vg_replace_malloc.c:270)
==22060==    by 0x4E6D79D: evhttp_connection_cb (http.c:1460)
==22060==    by 0x4E4D18D: bufferevent_writecb (bufferevent_sock.c:252)
==22060==    by 0x4E52481: event_process_active_single_queue (event.c:1532)
==22060==    by 0x4E52D66: event_base_loop (event.c:1617)