fix(showcmd): assert failure with cmdheight=0 (#21536)

src/nvim/normal.c

@@ -1990,6 +1990,9 @@ static void display_showcmd(void)
     return;
   }
   // 'showcmdloc' is "last" or empty
+  if (p_ch == 0 && !ui_has(kUIMessages)) {
+    return;
+  }
 
   if (ui_has(kUIMessages)) {
     MAXSIZE_TEMP_ARRAY(content, 1);

test/functional/ui/cmdline_spec.lua

@@ -1351,4 +1351,17 @@ describe('cmdheight=0', function()
       {1:~                             }│{1:~                            }|
     ]])
   end)
+
+  it('no assert failure with showcmd', function()
+    command('set showcmd cmdheight=0')
+    feed('d')
+    screen:expect([[
+      ^                         |
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+    ]])
+    assert_alive()
+  end)
 end)