patch 8.1.0437: may access freed memory when syntax HL times out

Problem:    May access freed memory when syntax HL times out. (Philipp Gesang)
Solution:   Clear b_sst_first when clearing b_sst_array.

src/version.c

@@ -794,6 +794,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    437,
 /**/
     436,
 /**/