tests: fix memory leak in test-bus-marshal

tests: fix memory leak in test-bus-marshal

Fixes:
```
$ ./configure ... --enable-dbus
$ make
$ make valgrind-tests TESTS=test-bus-marshal
...
==25301== 51 bytes in 1 blocks are definitely lost in loss record 7 of 18
==25301==    at 0x4C2DD9F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==25301==    by 0x5496B8C: ??? (in /lib/x86_64-linux-gnu/libdbus-1.so.3.14.3)
==25301==    by 0x54973E3: _dbus_string_append_printf_valist (in /lib/x86_64-linux-gnu/libdbus-1.so.3.14.3)
==25301==    by 0x547E5C2: _dbus_set_error_valist (in /lib/x86_64-linux-gnu/libdbus-1.so.3.14.3)
==25301==    by 0x547E73E: dbus_set_error (in /lib/x86_64-linux-gnu/libdbus-1.so.3.14.3)
==25301==    by 0x548969A: dbus_message_demarshal (in /lib/x86_64-linux-gnu/libdbus-1.so.3.14.3)
==25301==    by 0x115C1A: main (test-bus-marshal.c:244)
==25301==
```