disconnect: wipe out the keeps_speed time stamp

When closing a connection, the speedchecker's timestamp is now deleted
so that it cannot accidentally be used by a fresh connection on the same
handle when examining the transfer speed.

Bug: https://bugzilla.redhat.com/679709

RELEASE-NOTES

@@ -23,6 +23,7 @@ This release includes the following bugfixes:
  o GnuTLS handshake: fix timeout
  o cyassl: build without filesystem
  o HTTPS over HTTP proxy using the multi interface
+ o speedcheck: invalid timeout event on a reused handle
 
 This release includes the following known bugs:
 

lib/url.c

@@ -2675,6 +2675,7 @@ CURLcode Curl_disconnect(struct connectdata *conn, bool dead_connection)
 
   conn_free(conn);
   data->state.current_conn = NULL;
+  Curl_speedinit(data);
 
   return CURLE_OK;
 }