patch 8.0.1251: invalid expressin passed to WaitFor()

Problem:    Invalid expressin passed to WaitFor().
Solution:   Check if the variable exists.

Author: brammool

src/testdir/test_clientserver.vim

@@ -42,7 +42,7 @@ func Test_client_server()
   call remote_foreground(name)
 
   call remote_send(name, ":let testvar = 'yes'\<CR>")
-  call WaitFor('remote_expr("' . name . '", "testvar", "", 1) == "yes"')
+  call WaitFor('remote_expr("' . name . '", "exists(\"testvar\") ? testvar : \"\"", "", 1) == "yes"')
   call assert_equal('yes', remote_expr(name, "testvar", "", 2))
 
   if has('unix') && has('gui') && !has('gui_running')

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1251,
 /**/
     1250,
 /**/