patch 8.2.0644: insufficient testing for invalid function arguments

Problem:    Insufficient testing for invalid function arguments.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5988)
diff --git a/src/testdir/test_reltime.vim b/src/testdir/test_reltime.vim
index e25c3ce..f4ce5de 100644
--- a/src/testdir/test_reltime.vim
+++ b/src/testdir/test_reltime.vim
@@ -23,4 +23,9 @@
   call assert_true(reltimestr(differs) != '0.0')
   call assert_true(reltimefloat(differs) < 0.1)
   call assert_true(reltimefloat(differs) > 0.0)
+
+  call assert_equal(0, reltime({}))
+  call assert_equal(0, reltime({}, {}))
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab