patch 8.1.1592: may start file dialog while exiting

Problem:    May start file dialog while exiting.
Solution:   Ignore the "browse" modifier when exiting. (Ozaki Kiichi,
            closes #4582
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 78fe178..0eaa662 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3186,7 +3186,7 @@
 
     ffname = eap->arg;
 #ifdef FEAT_BROWSE
-    if (cmdmod.browse)
+    if (cmdmod.browse && !exiting)
     {
 	browse_file = do_browse(BROWSE_SAVE, (char_u *)_("Save As"), ffname,
 						    NULL, NULL, NULL, curbuf);