patch 8.2.0615: regexp benchmark stest is old style

Problem:    Regexp benchmark stest is old style.
Solution:   Make it a new style test.  Fix using a NULL list.  Add more tests.
            (Yegappan Lakshmanan, closes #5963)
diff --git a/src/evalbuffer.c b/src/evalbuffer.c
index 2c20939..f11727c 100644
--- a/src/evalbuffer.c
+++ b/src/evalbuffer.c
@@ -176,6 +176,13 @@
     if (lines->v_type == VAR_LIST)
     {
 	l = lines->vval.v_list;
+	if (l == NULL || list_len(l) == 0)
+	{
+	    // set proper return code
+	    if (lnum > curbuf->b_ml.ml_line_count)
+		rettv->vval.v_number = 1;	// FAIL
+	    goto done;
+	}
 	range_list_materialize(l);
 	li = l->lv_first;
     }
@@ -251,6 +258,7 @@
 	update_topline();
     }
 
+done:
     if (!is_curbuf)
     {
 	curbuf = curbuf_save;