patch 8.1.0936: may leak memory when using 'vartabstop'

Problem:    May leak memory when using 'vartabstop'. (Kuang-che Wu)
Solution:   Fix handling allocated memory for 'vartabstop'. (closes #3976)
diff --git a/src/buffer.c b/src/buffer.c
index db0c591..49d38ed 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2170,9 +2170,7 @@
 	vim_free(buf->b_p_vsts_array);
     buf->b_p_vsts_array = NULL;
     clear_string_option(&buf->b_p_vts);
-    if (buf->b_p_vts_array)
-	vim_free(buf->b_p_vts_array);
-    buf->b_p_vts_array = NULL;
+    VIM_CLEAR(buf->b_p_vts_array);
 #endif
 #ifdef FEAT_KEYMAP
     clear_string_option(&buf->b_p_keymap);