fix(ui): "resize -1" with cmdheight=0 #24758

Problem:
Crash from:

    set cmdheight=0 redrawdebug=invalid
    resize -1

Solution:
Do not invalidate first `p_ch` `msg_grid` rows in `update_screen` when
scrolling the screen down after displaying a message, because they may
be used later for drawing cmdline.
Fixes #22154

src/nvim/drawscreen.c

@@ -471,7 +471,7 @@ int update_screen(void)
       // non-displayed part of msg_grid is considered invalid.
       for (int i = 0; i < MIN(msg_scrollsize(), msg_grid.rows); i++) {
         grid_clear_line(&msg_grid, msg_grid.line_offset[i],
-                        msg_grid.cols, false);
+                        msg_grid.cols, i < p_ch);
       }
     }
     msg_grid.throttled = false;

test/functional/ui/cmdline_spec.lua

@@ -1021,6 +1021,26 @@ describe('cmdheight=0', function()
     screen:attach()
   end)
 
+  it("with redrawdebug=invalid resize -1", function()
+    command("set redrawdebug=invalid cmdheight=0 noruler laststatus=0")
+    screen:expect{grid=[[
+      ^                         |
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+    ]]}
+    feed(":resize -1<CR>")
+    screen:expect{grid=[[
+      ^                         |
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+                               |
+    ]]}
+    assert_alive()
+  end)
+
   it("with cmdheight=1 noruler laststatus=2", function()
     command("set cmdheight=1 noruler laststatus=2")
     screen:expect{grid=[[