vim-patch:8.1.2236: ml_get error if pattern matches beyond last line

Problem:    Ml_get error if pattern matches beyond last line.
Solution:   Adjust position if needed. (Christian Brabandt, closes )
bb26596242

src/nvim/ex_cmds.c

@@ -3524,6 +3524,11 @@ static buf_T *do_sub(exarg_T *eap, proftime_T timeout,
         // Note: If not first match on a line, column can't be known here
         current_match.start.lnum = sub_firstlnum;
 
+        // Match might be after the last line for "\n\zs" matching at
+        // the end of the last line.
+        if (lnum > curbuf->b_ml.ml_line_count) {
+          break;
+        }
         if (sub_firstline == NULL) {
           sub_firstline = vim_strsave(ml_get(sub_firstlnum));
         }

src/nvim/testdir/test_substitute.vim

@@ -149,6 +149,7 @@ func Run_SubCmd_Tests(tests)
   for t in a:tests
     let start = line('.') + 1
     let end = start + len(t[2]) - 1
+    " TODO: why is there a one second delay the first time we get here?
     exe "normal o" . t[0]
     call cursor(start, 1)
     exe t[1]
@@ -717,3 +718,12 @@ one two
 
   close!
 endfunc
+
+func Test_sub_beyond_end()
+  new
+  call setline(1, '#')
+  let @/ = '^#\n\zs'
+  s///e
+  call assert_equal('#', getline(1))
+  bwipe!
+endfunc