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.

Author: brammool

src/syntax.c

@@ -10044,11 +10044,11 @@ highlight_changed(void)
 #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

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1621,
 /**/
     1620,
 /**/