win_line: Fix crash with 'rightleft' in :terminal #11460

fixes #11438

Backtrace:

    0  schar_from_ascii ( p=0x801cc9e112c3 <error: Cannot access memory at address 0x801cc9e112c3>, c=32 ' ') at ../src/nvim/screen.c:5263
    1  0x00007f31460eccc5 in win_line (wp=wp@entry=0x7fffc9df6230, lnum=lnum@entry=11, startrow=startrow@entry=10, endrow=41, nochange=false, number_only=number_only@entry=false) at ../src/nvim/screen.c:4025
    2  0x00007f31460eed8e in win_update (wp=wp@entry=0x7fffc9df6230) at ../src/nvim/screen.c:1403
    3  0x00007f31460f011f in update_screen (type=<optimized out>) at ../src/nvim/screen.c:502
    4  0x00007f3146138ef4 in normal_redraw (s=s@entry=0x7fffd0a5f700) at ../src/nvim/normal.c:1247
    5  0x00007f314613b159 in normal_check (state=0x7fffd0a5f700) at ../src/nvim/normal.c:1324
    6  0x00007f31460accfe in state_enter (s=0x7fffd0a5f700) at ../src/nvim/state.c:28
    7  0x00007f3146143099 in normal_enter (cmdwin=<optimized out>, noexmode=<optimized out>) at ../src/nvim/normal.c:463
    8  0x00007f314618b541 in main (argc=<optimized out>, argv=<optimized out>) at ../src/nvim/main.c:580

1 files changed, 17 insertions, 0 deletions

diff --git a/test/functional/terminal/buffer_spec.lua b/test/functional/terminal/buffer_spec.lua
index 7560b0e872..2fc7a021cb 100644
--- a/test/functional/terminal/buffer_spec.lua
+++ b/test/functional/terminal/buffer_spec.lua
@@ -5,6 +5,7 @@ local wait = helpers.wait
 local eval, feed_command, source = helpers.eval, helpers.feed_command, helpers.source
 local eq, neq = helpers.eq, helpers.neq
 local write_file = helpers.write_file
+local command= helpers.command
 
 describe(':terminal buffer', function()
   local screen
@@ -224,6 +225,22 @@ describe(':terminal buffer', function()
       neq('terminal', eval('&buftype'))
     end)
   end)
+
+  it('it works with set rightleft #11438', function()
+    local columns = eval('&columns')
+    feed(string.rep('a', columns))
+    command('set rightleft')
+    screen:expect([[
+                                               ydaer ytt|
+      {1:a}aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa|
+                                                        |
+                                                        |
+                                                        |
+                                                        |
+      {3:-- TERMINAL --}                                    |
+    ]])
+    command('bdelete!')
+  end)
 end)
 
 describe('No heap-buffer-overflow when using', function()