patch 8.1.1603: crash when using unknown highlighting in text property

Problem:    Crash when using unknown highlighting in text property.
Solution:   Check for zero highlight ID.
diff --git a/src/version.c b/src/version.c
index d44171d..26a9bf5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1603,
+/**/
     1602,
 /**/
     1601,