patch 8.2.0909: cannot go back to the previous local directory

Problem:    Cannot go back to the previous local directory.
Solution:   Add "tcd -" and "lcd -". (Yegappan Lakshmanan, closes #4362)
diff --git a/src/window.c b/src/window.c
index ede2c6f..931d86b 100644
--- a/src/window.c
+++ b/src/window.c
@@ -3809,6 +3809,7 @@
 #endif
 
     vim_free(tp->tp_localdir);
+    vim_free(tp->tp_prevdir);
 
 #ifdef FEAT_PYTHON
     python_tabpage_free(tp);
@@ -4974,6 +4975,7 @@
 	vim_free(wp->w_tagstack[i].user_data);
     }
     vim_free(wp->w_localdir);
+    vim_free(wp->w_prevdir);
 
     // Remove the window from the b_wininfo lists, it may happen that the
     // freed memory is re-used for another window.