Fixes a bug likely introduced in
d396819 (in 1.8.1) whereby
‘proxy_settings.on_status’ would be left uninitialized when using the
‘http-parser’ backend, eventually leading to a segfault in
‘http_parser_execute’.  Valgrind would report use of the uninitialized
value like so:

   Conditional jump or move depends on uninitialised value(s)
      at 0x50CD533: http_parser_execute (http_parser.c:910)
      by 0x4928504: git_http_parser_execute (httpparser.c:82)
      by 0x4925C42: client_read_and_parse (httpclient.c:1178)
      by 0x4926F27: git_http_client_read_response (httpclient.c:1458)
      by 0x49255FE: http_stream_read (http.c:427)
      by 0x4929B90: git_smart__recv (smart.c:29)
      by 0x492C147: git_smart__store_refs (smart_protocol.c:58)
      by 0x4929F6C: git_smart__connect (smart.c:171)
      by 0x4904DCE: git_remote_connect_ext (remote.c:963)
      by 0x48A15D2: clone_into (clone.c:449)
      by 0x48A15D2: git__clone (clone.c:546)
      by 0x4010E9: main (libgit2-proxy.c:20)