Bug div by zero in brush preview

[sentry]

Sentry Issue: ASEPRITE-177

Not a new crash or regression.

Thread 22940 Crashed:
0   aseprite.exe                    0x7ff77edab0aa      app::Site::grid (site.cpp:95)
1   aseprite.exe                    0x7ff77f0ca64c      app::PreviewToolLoopImpl::PreviewToolLoopImpl (tool_loop_impl.cpp:950)
2   aseprite.exe                    0x7ff77f0cd56d      app::create_tool_loop_preview (tool_loop_impl.cpp:1053)
3   aseprite.exe                    0x7ff77efa9c14      app::BrushPreview::show (brush_preview.cpp:368)
4   aseprite.exe                    0x7ff77f0da637      app::StateWithWheelBehavior::onSetCursor (state_with_wheel_behavior.cpp:590)
5   aseprite.exe                    0x7ff77efd3f7a      app::StandbyState::onSetCursor (standby_state.cpp:488)
6   aseprite.exe                    0x7ff77ede6133      app::Editor::setCursor (editor.cpp:2376)
7   aseprite.exe                    0x7ff77ede3d75      app::Editor::onProcessMessage (editor.cpp:2090)
8   aseprite.exe                    0x7ff77f01c0a5      app::`anonymous namespace'::AppEditor::onProcessMessage (doc_view.cpp:168)
9   aseprite.exe                    0x7ff77f1aa1c1      ui::Manager::sendMessageToWidget (manager.cpp:1987)
10  aseprite.exe                    0x7ff77f1a98b5      ui::Manager::pumpQueue (manager.cpp:1862)
11  aseprite.exe                    0x7ff77f1a691d      ui::Manager::dispatchMessages (manager.cpp:842)
12  aseprite.exe                    0x7ff77f1aa059      ui::Manager::run (manager.cpp:291)
13  aseprite.exe                    0x7ff77ecf294a      app::App::run (app.cpp:546)
14  aseprite.exe                    0x7ff77ecd678d      app_main (main.cpp:148)
15  aseprite.exe                    0x7ff77f425ea4      wWinMain (main.cpp:40)
16  aseprite.exe                    0x7ff77f39a56d      [inlined] invoke_main (exe_common.inl:118)
17  aseprite.exe                    0x7ff77f39a56d      __scrt_common_main_seh (exe_common.inl:288)
18  KERNEL32.DLL                    0x7ffae6bb7343      BaseThreadInitThunk
19  ntdll.dll                       0x7ffae70626b0      RtlUserThreadStart