http: do not call deferred readcb if readcb is not set Otherwise evhttp_read_cb can be called with invalid connection state: http/read_on_write_error: [forking] [msg] libevent using: kqueue FAIL ../test/regress_http.c:4079: assert(req) FAIL ../test/regress_http.c:4087: assert(req)[err] evhttp_read_cb: illegal connection state 0