patch 8.0.1425: execute() does not work in completion of user command

Problem:    execute() does not work in completion of user command. (thinca)
Solution:   Switch off redir_off and restore it. (Ozaki Kiichi, closes #2492)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 764a4db..0300efe 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -2886,6 +2886,7 @@
     int		save_emsg_silent = emsg_silent;
     int		save_emsg_noredir = emsg_noredir;
     int		save_redir_execute = redir_execute;
+    int		save_redir_off = redir_off;
     garray_T	save_ga;
 
     rettv->vval.v_string = NULL;
@@ -2928,6 +2929,7 @@
 	save_ga = redir_execute_ga;
     ga_init2(&redir_execute_ga, (int)sizeof(char), 500);
     redir_execute = TRUE;
+    redir_off = FALSE;
 
     if (cmd != NULL)
 	do_cmdline_cmd(cmd);
@@ -2958,6 +2960,7 @@
     redir_execute = save_redir_execute;
     if (redir_execute)
 	redir_execute_ga = save_ga;
+    redir_off = save_redir_off;
 
     /* "silent reg" or "silent echo x" leaves msg_col somewhere in the
      * line.  Put it back in the first column. */