[mod_fastcgi] fix memleak with FastCGI auth,resp (fixes #2894)

fix memleak in mod_fastcgi when FastCGI is used for both authentication
and response on the same request

(thx rschmid)

x-ref:
  "Memory leak if two fcgi calls with one request (authentication and response)"
  https://redmine.lighttpd.net/issues/2894

src/mod_fastcgi.c

@@ -515,7 +515,12 @@ static handler_t fcgi_check_extension(server *srv, connection *con, void *p_d, i
 		hctx->opts.pdata = hctx;
 		hctx->stdin_append = fcgi_stdin_append;
 		hctx->create_env = fcgi_create_env;
-		hctx->rb = chunkqueue_init();
+		if (!hctx->rb) {
+			hctx->rb = chunkqueue_init();
+		}
+		else {
+			chunkqueue_reset(hctx->rb);
+		}
 	}
 
 	return HANDLER_GO_ON;