patch 9.0.0991: crash when reading help index with various options set

Problem:    Crash when reading help index with various options set. (Marius
            Gedminas)
Solution:   Do not set wlv.c_extra to NUL when wlv.p_extra is NULL.
            (closes #11651)
diff --git a/src/version.c b/src/version.c
index f9b1e6c..07a6093 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    991,
+/**/
     990,
 /**/
     989,