steam-http: fixed requests being freed with steam_http_req_resend()

When steam_http_req_resend() is called, the SteamHttpReq must not be
freed after the SteamHttpFunc has been executed. This causes two major
issues:

  1. The SteamHttpReq is resent, then freed before completion
  2. The freed SteamHttpReq is left in the request queue

Both issues can and will cause segfaults to occur.