dump: Fix g_array_unref(NULL) in dump-guest-memory

When dump_init()'s check for non-zero @length fails, dump_cleanup()
passes null s->string_table_buf to g_array_unref(), which spews "GLib:
g_array_unref: assertion 'array' failed" to stderr.

Guard the g_array_unref().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20231031104531.3169721-3-armbru@redhat.com>