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.
diff --git a/src/syntax.c b/src/syntax.c
index 637a450..7303575 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -9999,7 +9999,7 @@
     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. */
diff --git a/src/version.c b/src/version.c
index c1260ec..d820595 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    970,
+/**/
     969,
 /**/
     968,