vim-patch:8.1.0425: ml_get error and crash with appendbufline()

Problem:    ml_get error and crash with appendbufline(). (Masashi Iizuka)
Solution:   Set per-window buffer info. (Hirohito Higashi, closes vim/vim#3455)

9cea87c577

Co-authored-by: Bram Moolenaar <Bram@vim.org>
(cherry picked from commit c6f0a19206)

src/nvim/buffer.c

@@ -1728,6 +1728,9 @@ void enter_buffer(buf_T *buf)
   // mark cursor position as being invalid
   curwin->w_valid = 0;
 
+  buflist_setfpos(curbuf, curwin, curbuf->b_last_cursor.mark.lnum,
+                  curbuf->b_last_cursor.mark.col, true);
+
   // Make sure the buffer is loaded.
   if (curbuf->b_ml.ml_mfp == NULL) {    // need to load the file
     // If there is no filetype, allow for detecting one.  Esp. useful for

test/old/testdir/test_bufline.vim

@@ -158,6 +158,33 @@ func Test_appendbufline()
   exe "bwipe! " . b
 endfunc
 
+func Test_appendbufline_no_E315()
+  let after = [
+    \ 'set stl=%f ls=2',
+    \ 'new',
+    \ 'let buf = bufnr("%")',
+    \ 'quit',
+    \ 'vsp',
+    \ 'exec "buffer" buf',
+    \ 'wincmd w',
+    \ 'call appendbufline(buf, 0, "abc")',
+    \ 'redraw',
+    \ 'while getbufline(buf, 1)[0] =~ "^\\s*$"',
+    \ '  sleep 10m',
+    \ 'endwhile',
+    \ 'au VimLeavePre * call writefile([v:errmsg], "Xerror")',
+    \ 'au VimLeavePre * call writefile(["done"], "Xdone")',
+    \ 'qall!',
+    \ ]
+  if !RunVim([], after, '--clean')
+    return
+  endif
+  call assert_notmatch("^E315:", readfile("Xerror")[0])
+  call assert_equal("done", readfile("Xdone")[0])
+  call delete("Xerror")
+  call delete("Xdone")
+endfunc
+
 func Test_deletebufline()
   new
   let b = bufnr('%')