patch 8.0.0970: passing invalid highlight id

Problem:    if there is no StatusLine highlighting and there is StatusLineNC
            or StatusLineTermNC highlighting then an invalid highlight id is
            passed to combine_stl_hlt(). (Coverity)
Solution:   Check id_S to be -1 instead of zero.

src/syntax.c

@@ -9999,7 +9999,7 @@ highlight_changed(void)
     if (ga_grow(&highlight_ga, 28) == FAIL)
 	return FAIL;
     hlcnt = highlight_ga.ga_len;
-    if (id_S == 0)
+    if (id_S == -1)
     {
 	/* Make sure id_S is always valid to simplify code below. Use the last
 	 * entry. */

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    970,
 /**/
     969,
 /**/