patch 8.0.0988: warning from Covscan about using NULL pointer

Problem:    Warning from Covscan about using NULL pointer.
Solution:   Add extra check for NULL. (zdohnal)
diff --git a/src/version.c b/src/version.c
index 6d2de86..83cb3d9 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    988,
+/**/
     987,
 /**/
     986,