Description: In an older version of libgit2 in git_object_lookup_prefix was a check that repo is valid, but now there is repo->oid_type in the git_object_lookup and must be checked as well

Add assert

Fixes a bug likely introduced ind396819 (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 uninitializedvalue 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)

Co-authored-by: Edward Thomson <ethomson@edwardthomson.com>