* Avoid calling ap_proxy_http_cleanup twice as this releases a connection

  from the connection pool twice. This causes this connection to be present
  in the connection pool twice. Thus it may be used by different threads
  at the same time which causes many troubles (segfaults in this case).
  Furthermore implement a logic to prevent double releases to the connection
  pool if they are triggered by buggy code and log an error message in this
  case.

  - mod_proxy_http.c: remove double calls to ap_proxy_http_cleanup
  - proxy_util.c: Add logic to prevent double releases of a
    connection to the connection pool.

PR: 38793


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@394088 13f79535-47bb-0310-9956-ffa450edef68