add some (void) casts for unused variables

All these places already have an XXX marker, so we know we need to fix
them later. Let's not clutter the static analysis results with them.

bufferevent_filter.c

@@ -428,6 +428,7 @@ be_filter_readcb(struct bufferevent *underlying, void *_me)
 
 	/* XXXX use return value */
 	res = be_filter_process_input(bevf, state, &processed_any);
+	(void)res;
 
 	/* XXX This should be in process_input, not here.  There are
 	 * other places that can call process-input, and they should

bufferevent_openssl.c

@@ -1013,6 +1013,7 @@ be_openssl_outbuf_cb(struct evbuffer *buf,
 		consider_writing(bev_ssl);
 	}
 	/* XXX Handle r < 0 */
+        (void)r;
 }
 
 

bufferevent_sock.c

@@ -451,6 +451,7 @@ bufferevent_connect_getaddrinfo_cb(int result, struct evutil_addrinfo *ai,
 	/* XXX use the other addrinfos? */
 	/* XXX use this return value */
 	r = bufferevent_socket_connect(bev, ai->ai_addr, (int)ai->ai_addrlen);
+	(void)r;
 	_bufferevent_decref_and_unlock(bev);
 	evutil_freeaddrinfo(ai);
 }