fix(messages): avoid crash with :intro and ch=0 (#28343)

This just copies code from msg_start() to wait_return().  Not sure if
there is a better place to put such a block.

src/nvim/message.c

@@ -1191,6 +1191,14 @@ void wait_return(int redraw)
       check_timestamps(false);
     }
 
+    // if cmdheight=0, we need to scroll in the first line of msg_grid upon the screen
+    if (p_ch == 0 && !ui_has(kUIMessages) && !msg_scrolled) {
+      msg_grid_validate();
+      msg_scroll_up(false, true);
+      msg_scrolled++;
+      cmdline_row = Rows - 1;
+    }
+
     hit_return_msg(true);
 
     do {

test/functional/ui/messages_spec.lua

@@ -1536,6 +1536,35 @@ vimComment     xxx match /\s"[^\-:.%#=*].*$/ms=s+1,lc=1  excludenl contains=@vim
                                                                   |*4
     ]])
   end)
+
+  it('supports :intro with cmdheight=0 #26505', function()
+    screen:try_resize(80, 24)
+    command('set cmdheight=0')
+    feed(':intro<CR>')
+    screen:expect([[
+                                                                                      |*5
+      {MATCH:.*}|
+                                                                                      |
+                        Nvim is open source and freely distributable                  |
+                                  https://neovim.io/#chat                             |
+                                                                                      |
+                       type  :help nvim{18:<Enter>}       if you are new!                  |
+                       type  :checkhealth{18:<Enter>}     to optimize Nvim                 |
+                       type  :q{18:<Enter>}               to exit                          |
+                       type  :help{18:<Enter>}            for help                         |
+                                                                                      |
+      {MATCH: +}type  :help news{18:<Enter>} to see changes in v{MATCH:%d+%.%d+ +}|
+                                                                                      |
+                               Help poor children in Uganda!                          |
+                       type  :help iccf{18:<Enter>}       for information                  |
+                                                                                      |*2
+      {3:                                                                                }|
+                                                                                      |
+      {6:Press ENTER or type command to continue}^                                         |
+    ]])
+    feed('<CR>')
+    assert_alive()
+  end)
 end)
 
 it('calling screenstring() after redrawing between messages without UI #20999', function()