vim-patch:8.1.1812: reading a truncted undo file hangs Vim

Problem:    Reading a truncted undo file hangs Vim.
Solution:   Check for reading EOF. (closes vim/vim#4769)
fb06d767a8

src/nvim/testdir/test_undo.vim

@@ -364,6 +364,25 @@ func Test_wundo_errors()
   bwipe!
 endfunc
 
+" Check that reading a truncted undo file doesn't hang.
+func Test_undofile_truncated()
+  throw 'skipped: TODO: '
+  new
+  call setline(1, 'hello')
+  set ul=100
+  wundo Xundofile
+  let contents = readfile('Xundofile', 'B')
+
+  " try several sizes
+  for size in range(20, 500, 33)
+    call writefile(contents[0:size], 'Xundofile')
+    call assert_fails('rundo Xundofile', 'E825:')
+  endfor
+
+  bwipe!
+"  call delete('Xundofile')
+endfunc
+
 func Test_rundo_errors()
   call assert_fails('rundo XfileDoesNotExist', 'E822:')
 

src/nvim/undo.c

@@ -878,7 +878,12 @@ static u_header_T *unserialize_uhp(bufinfo_T *bi,
   for (;; ) {
     int len = undo_read_byte(bi);
 
-    if (len == 0 || len == EOF) {
+    if (len == EOF) {
+      corruption_error("truncated", file_name);
+      u_free_uhp(uhp);
+      return NULL;
+    }
+    if (len == 0) {
       break;
     }
     int what = undo_read_byte(bi);