patch 8.1.0581: double free without the text properties feature

Problem:    Double free without the text properties feature.
Solution:   Reset the dirty flag.
diff --git a/src/version.c b/src/version.c
index 9f097af..9a44976 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    581,
+/**/
     580,
 /**/
     579,