Fix sanity check of door_secret_move6() and door_secret_die().

After this change they match the sanity checks in baseq2 and xatrix.
This was reported by maraakate in yquake2 issue #160.

src/g_func.c

@@ -3745,6 +3745,11 @@ door_secret_move5(edict_t *self)
 void
 door_secret_move6(edict_t *self)
 {
+	if (!self)
+	{
+		return;
+	}
+
 	Move_Calc(self, vec3_origin, door_secret_done);
 }
 
@@ -3806,7 +3811,7 @@ door_secret_die(edict_t *self, edict_t *inflictor /* unused */,
 		edict_t *attacker, int damage /* unused */,
 		vec3_t point /* unused */)
 {
-	if (!self)
+	if (!self || !attacker)
 	{
 		return;
 	}