vim-patch:8.2.4281: using freed memory with :lopen and :bwipe

Problem:    Using freed memory with :lopen and :bwipe.
Solution:   Do not use a wiped out buffer.
9b4a80a665

Cherry-pick some indent changes from patch 8.2.1432.

src/nvim/buffer.c

@@ -1486,8 +1486,15 @@ void set_curbuf(buf_T *buf, int action)
   // An autocommand may have deleted "buf", already entered it (e.g., when
   // it did ":bunload") or aborted the script processing!
   // If curwin->w_buffer is null, enter_buffer() will make it valid again
-  if ((buf_valid(buf) && buf != curbuf && !aborting()) || curwin->w_buffer == NULL) {
+  bool valid = buf_valid(buf);
+  if ((valid && buf != curbuf && !aborting()) || curwin->w_buffer == NULL) {
+    // If the buffer is not valid but curwin->w_buffer is NULL we must
+    // enter some buffer.  Using the last one is hopefully OK.
+    if (!valid) {
+      enter_buffer(lastbuf);
+    } else {
       enter_buffer(buf);
+    }
     if (old_tw != curbuf->b_p_tw) {
       check_colorcolumn(curwin);
     }

src/nvim/testdir/test_quickfix.vim

@@ -891,6 +891,7 @@ func Test_locationlist_curwin_was_closed()
   call assert_fails('lrewind', 'E924:')
 
   augroup! testgroup
+  delfunc R
 endfunc
 
 func Test_locationlist_cross_tab_jump()
@@ -5489,4 +5490,20 @@ func Test_two_qf_windows()
   %bw!
 endfunc
 
+" Weird sequence of commands that caused entering a wiped-out buffer
+func Test_lopen_bwipe()
+  func R()
+    silent! tab lopen
+    e x
+    silent! lfile
+  endfunc
+
+  cal R()
+  cal R()
+  cal R()
+  bw!
+  delfunc R
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab