patch 8.1.1259: crash when exiting early

Problem:    Crash when exiting early. (Ralf Schandl)
Solution:   Only pop/push the title when it was set. (closes #4334)
diff --git a/src/os_unix.c b/src/os_unix.c
index a78b330..ba5d386 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -2205,14 +2205,19 @@
     void
 mch_restore_title(int which)
 {
+    int	do_push_pop = did_set_title || did_set_icon;
+
     /* only restore the title or icon when it has been set */
     mch_settitle(((which & SAVE_RESTORE_TITLE) && did_set_title) ?
 			(oldtitle ? oldtitle : p_titleold) : NULL,
 	       ((which & SAVE_RESTORE_ICON) && did_set_icon) ? oldicon : NULL);
 
-    // pop and push from/to the stack
-    term_pop_title(which);
-    term_push_title(which);
+    if (do_push_pop)
+    {
+	// pop and push from/to the stack
+	term_pop_title(which);
+	term_push_title(which);
+    }
 }
 
 #endif /* FEAT_TITLE */