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>
diff --git a/dump/dump.c b/dump/dump.c
index 9cdb4a2..24c829e 100644
--- a/dump/dump.c
+++ b/dump/dump.c
@@ -100,7 +100,7 @@
     memory_mapping_list_free(&s->list);
     close(s->fd);
     g_free(s->guest_note);
-    g_array_unref(s->string_table_buf);
+    g_clear_pointer(&s->string_table_buf, g_array_unref);
     s->guest_note = NULL;
     if (s->resume) {
         if (s->detached) {