http: avoid leaking of fd in evhttp_connection_free()

http: avoid leaking of fd in evhttp_connection_free()

Since we do close fd there if we don't have BEV_OPT_CLOSE_ON_FREE, and
evcon->fd can be incorrect (non -1), so just get it from the underlying
bufferevent to fix this.

And after this patch the following tests report 0 instead of 2307 fd leaks:
$ valgrind --leak-check=full --show-reachable=yes --track-fds=yes --error-exitcode=1 regress --no-fork http/cancel..
==11299== FILE DESCRIPTORS: 3 open at exit.

And this is stdin/stderr/stdout.