Crash when saving an RSI that is missing a png

[ShadowCommander]

Crashes when saving an RSI that contains a state that has been deleted/does not exist.

Reproduction:

• Create new RSI
• Add a sprite in the editor
• Save
• Close the tab
• Delete the .png from the newly created RSI folder
• Open the RSI in the editor
• Save
• Editor crashes

Unhandled exception. System.ArgumentOutOfRangeException: Parameter "width" (System.Int32) must be greater than 0, was 0 (Parameter 'width')
   at SixLabors.ThrowHelper.ThrowArgumentOutOfRangeException(String name, String message)
   at SixLabors.ThrowHelper.ThrowArgumentOutOfRangeExceptionForMustBeGreaterThan[T](T value, T minimum, String name)
   at SixLabors.ImageSharp.ImageFrame`1..ctor(Configuration configuration, Int32 width, Int32 height, TPixel backgroundColor, ImageFrameMetadata metadata)
   at SixLabors.ImageSharp.ImageFrame`1..ctor(Configuration configuration, Int32 width, Int32 height, TPixel backgroundColor)
   at SixLabors.ImageSharp.ImageFrameCollection`1..ctor(Image`1 parent, Int32 width, Int32 height, TPixel backgroundColor)
   at SixLabors.ImageSharp.Image`1..ctor(Configuration configuration, Int32 width, Int32 height, ImageMetadata metadata)
   at SixLabors.ImageSharp.Image`1..ctor(Configuration configuration, Int32 width, Int32 height)
   at SixLabors.ImageSharp.Image`1..ctor(Int32 width, Int32 height)
   at SpaceWizards.RsiLib.RSI.RsiState.GetFullImage(RsiSize size) in /home/runner/work/RSIEdit/RSIEdit/RSI.NET/SpaceWizards.RsiLib/RSI/RsiState.cs:line 99
   at SpaceWizards.RsiLib.RSI.Rsi.SaveImagesToFolder(String rsiFolder) in /home/runner/work/RSIEdit/RSIEdit/RSI.NET/SpaceWizards.RsiLib/RSI/Rsi.cs:line 118
   at SpaceWizards.RsiLib.RSI.Rsi.SaveToFolder(String rsiFolder) in /home/runner/work/RSIEdit/RSIEdit/RSI.NET/SpaceWizards.RsiLib/RSI/Rsi.cs:line 108
   at Editor.ViewModels.RsiItemViewModel.Save() in /home/runner/work/RSIEdit/RSIEdit/Editor/ViewModels/RsiItemViewModel.cs:line 628
   at Editor.ViewModels.MainWindowViewModel.Save() in /home/runner/work/RSIEdit/RSIEdit/Editor/ViewModels/MainWindowViewModel.cs:line 268
   at Avalonia.Data.Converters.MethodToCommandConverter.Execute(Object parameter) in /_/src/Avalonia.Base/Data/Converters/MethodToCommandConverter.cs:line 82
   at Avalonia.Input.KeyBinding.TryHandle(KeyEventArgs args) in /_/src/Avalonia.Input/KeyBinding.cs:line 41
   at Avalonia.Input.KeyboardDevice.ProcessRawEvent(RawInputEventArgs e) in /_/src/Avalonia.Input/KeyboardDevice.cs:line 241
   at Avalonia.Input.InputManager.ProcessInput(RawInputEventArgs e) in /_/src/Avalonia.Input/InputManager.cs:line 37
   at Avalonia.Win32.WindowImpl.AppWndProc(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam) in /_/src/Windows/Avalonia.Win32/WindowImpl.AppWndProc.cs:line 524
   at Avalonia.Win32.WindowImpl.WndProc(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam) in /_/src/Windows/Avalonia.Win32/WindowImpl.WndProc.cs:line 33
   at Avalonia.Win32.Interop.UnmanagedMethods.DispatchMessage(MSG& lpmsg)
   at Avalonia.Win32.Win32Platform.RunLoop(CancellationToken cancellationToken) in /_/src/Windows/Avalonia.Win32/Win32Platform.cs:line 210
   at Avalonia.Threading.Dispatcher.MainLoop(CancellationToken cancellationToken) in /_/src/Avalonia.Base/Threading/Dispatcher.cs:line 65
   at Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.Start(String[] args) in /_/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs:line 120
   at Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime[T](T builder, String[] args, ShutdownMode shutdownMode) in /_/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs:line 209
   at Editor.Program.Main(String[] args) in /home/runner/work/RSIEdit/RSIEdit/Editor/Program.cs:line 20