systemd: give socket-activated endpoints a default endpoint_cfg

Sockets passed by systemd socket activation never get endpoint_cfg set,
but inc_listen_connections() dereferences it unconditionally at accept()
time. The first connection to a socket-activated unit therefore crashed
the daemon with a NULL pointer dereference, and probe_client_protocol()
dereferences it as well for the proxyprotocol check.

Verified with a stub sd_listen_fds(): sslh-select segfaults (SIGSEGV) on
the first connection to an activated socket before the fix, and forwards
the connection normally after it.

Assign a static zeroed sslhcfg_listen_item so these sockets behave as if
configured with no max_connections limit and no proxyprotocol.

common.c

@@ -74,6 +74,13 @@ int get_fd_sockets(struct listen_endpoint *sockfd[])
     int sd = 0;
 
 #ifdef SYSTEMD
+    /* Sockets passed by systemd socket activation do not correspond to a
+     * `listen` entry in the configuration, but code dereferences
+     * endpoint->endpoint_cfg unconditionally (e.g. inc_listen_connections()
+     * and probe_client_protocol()), so give them a default, empty
+     * configuration: no max_connections, no proxyprotocol. */
+    static struct sslhcfg_listen_item default_endpoint_cfg;
+
     sd = sd_listen_fds(0);
     if (sd < 0) {
       print_message(msg_system_error, "sd_listen_fds(): %s\n", strerror(-sd));
@@ -86,6 +93,7 @@ int get_fd_sockets(struct listen_endpoint *sockfd[])
       for (i = 0; i < sd; i++) {
         (*sockfd)[i].socketfd = SD_LISTEN_FDS_START + i;
         (*sockfd)[i].type = SOCK_STREAM;
+        (*sockfd)[i].endpoint_cfg = &default_endpoint_cfg;
       }
     }
 #endif