cover edge case of receiving the response to a request before the callback of sending the request. fixes assert

git-svn-id: http://svn.code.sf.net/p/libtorrent/code/trunk@10173 a83610d8-ad2a-0410-a6ab-fc0612d85776
merge changes from RC_1_0

git-svn-id: http://svn.code.sf.net/p/libtorrent/code/trunk@10176 a83610d8-ad2a-0410-a6ab-fc0612d85776