patch 8.1.0593: illegal memory access in libvterm test

Problem:    Illegal memory access in libvterm test.
Solution:   Fix off-by-one error.
diff --git a/src/version.c b/src/version.c
index 4e79d28..b1de233 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    593,
+/**/
     592,
 /**/
     591,