patch 8.0.1621: using invalid default value for highlight attribute

Problem:    Using invalid default value for highlight attribute.
Solution:   Use zero instead of -1.
diff --git a/src/syntax.c b/src/syntax.c
index d09359a..e945dac 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -10044,11 +10044,11 @@
 #ifdef USER_HIGHLIGHT
     char_u      userhl[10];
 # ifdef FEAT_STL_OPT
-    int		id_SNC = -1;
     int		id_S = -1;
+    int		id_SNC = 0;
 #  ifdef FEAT_TERMINAL
-    int		id_ST = -1;
-    int		id_STNC = -1;
+    int		id_ST = 0;
+    int		id_STNC = 0;
 #  endif
     int		hlcnt;
 # endif
diff --git a/src/version.c b/src/version.c
index acc535e..339ac90 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1621,
+/**/
     1620,
 /**/
     1619,