[core] fix removal of server.pid-file when testing (fixes #3115)

(thx dinoex)

fix removal of server.pid-file when testing config with lighttpd -tt
(regression in lighttpd 1.4.60, 1.4.61)

x-ref:
  "Test the configuration file while lighttpd is running."
  https://redmine.lighttpd.net/issues/3115

src/server.c

@@ -962,7 +962,7 @@ static int server_graceful_state_bg (server *srv) {
             close(pid_fd);
             pid_fd = -1;
     }
-    if (srv->srvconf.pid_file) buffer_clear(srv->srvconf.pid_file);
+    srv->srvconf.pid_file = NULL;
 
     /* (original process is backgrounded -- even if no active connections --
      *  to allow graceful shutdown tasks to be run by server and by modules) */
@@ -1024,7 +1024,7 @@ static void server_graceful_state (server *srv) {
         server_sockets_close(srv);
         remove_pid_file(srv);
         /*(prevent more removal attempts)*/
-        if (srv->srvconf.pid_file) buffer_clear(srv->srvconf.pid_file);
+        srv->srvconf.pid_file = NULL;
     }
 }
 
@@ -1163,7 +1163,7 @@ static int server_main_setup (server * const srv, int argc, char **argv) {
 	}
 
 	if (test_config) {
-		if (srv->srvconf.pid_file) buffer_clear(srv->srvconf.pid_file);
+		srv->srvconf.pid_file = NULL;
 		if (1 == test_config) {
 			printf("Syntax OK\n");
 		} else { /*(test_config > 1)*/
@@ -1186,7 +1186,7 @@ static int server_main_setup (server * const srv, int argc, char **argv) {
 		graceful_shutdown = 1;
 		srv->sockets_disabled = 2;
 		srv->srvconf.dont_daemonize = 1;
-		if (srv->srvconf.pid_file) buffer_clear(srv->srvconf.pid_file);
+		srv->srvconf.pid_file = NULL;
 		if (srv->srvconf.max_worker) {
 			srv->srvconf.max_worker = 0;
 			log_error(srv->errh, __FILE__, __LINE__,
@@ -1707,7 +1707,7 @@ static int server_main_setup (server * const srv, int argc, char **argv) {
 			close(pid_fd);
 			pid_fd = -1;
 		}
-		if (srv->srvconf.pid_file) buffer_clear(srv->srvconf.pid_file);
+		srv->srvconf.pid_file = NULL;
 
 		fdlog_pipes_abandon_pids();
 		srv->pid = getpid();