gdbus: Avoid printing null strings

This mostly affects the 2.56 branch, but, given that GCC 9 is being
stricter about passing null string pointers to printf-like functions, it
might make sense to proactively fix such calls.

gdbusauth.c: In function '_g_dbus_auth_run_server':
gdbusauth.c:1302:11: error: '%s' directive argument is null
[-Werror=format-overflow=]
 1302 |           debug_print ("SERVER: WaitingForBegin, read '%s'",
 line);
       |

gdbusmessage.c: In function ‘g_dbus_message_to_blob’:
gdbusmessage.c:2730:30: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
 2730 |       tupled_signature_str = g_strdup_printf ("(%s)", signature_str);
      |

gio/gdbusauth.c

@@ -1272,9 +1272,9 @@ _g_dbus_auth_run_server (GDBusAuth              *auth,
                                                     &line_length,
                                                     cancellable,
                                                     error);
-          debug_print ("SERVER: WaitingForBegin, read '%s'", line);
           if (line == NULL)
             goto out;
+          debug_print ("SERVER: WaitingForBegin, read '%s'", line);
           if (g_strcmp0 (line, "BEGIN") == 0)
             {
               /* YAY, done! */