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.
diff --git a/src/syntax.c b/src/syntax.c
index 8fb4e20..e54ec9a 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -1192,6 +1192,7 @@
 	for (p = block->b_sst_first; p != NULL; p = p->sst_next)
 	    clear_syn_state(p);
 	VIM_CLEAR(block->b_sst_array);
+	block->b_sst_first = NULL;
 	block->b_sst_len = 0;
     }
 }
@@ -1323,9 +1324,6 @@
     synstate_T	*p, *prev, *np;
     linenr_T	n;
 
-    if (block->b_sst_array == NULL)	/* nothing to do */
-	return;
-
     prev = NULL;
     for (p = block->b_sst_first; p != NULL; )
     {
@@ -1378,7 +1376,7 @@
     int		dist;
     int		retval = FALSE;
 
-    if (syn_block->b_sst_array == NULL || syn_block->b_sst_first == NULL)
+    if (syn_block->b_sst_first == NULL)
 	return retval;
 
     /* Compute normal distance between non-displayed entries. */