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/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,