Fix pad memory access error

Author: JimBobSquarePants

src/ImageSharp/Processing/Extensions/Transforms/PadExtensions.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
+
 namespace SixLabors.ImageSharp.Processing
 {
     /// <summary>
@@ -29,9 +31,11 @@ public static IImageProcessingContext Pad(this IImageProcessingContext source, i
         /// <returns>The <see cref="IImageProcessingContext"/> to allow chaining of operations.</returns>
         public static IImageProcessingContext Pad(this IImageProcessingContext source, int width, int height, Color color)
         {
+            Size size = source.GetCurrentSize();
             var options = new ResizeOptions
             {
-                Size = new Size(width, height),
+                // Prevent downsizing.
+                Size = new Size(Math.Max(width, size.Width), Math.Max(height, size.Height)),
                 Mode = ResizeMode.BoxPad,
                 Sampler = KnownResamplers.NearestNeighbor,
                 PadColor = color

src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeProcessor{TPixel}.cs

@@ -61,12 +61,9 @@ public void ApplyTransform<TResampler>(in TResampler sampler)
             Rectangle destinationRectangle = this.destinationRectangle;
             bool compand = this.options.Compand;
             bool premultiplyAlpha = this.options.PremultiplyAlpha;
-
-            this.options.PadColor = Color.GreenYellow;
-
+            TPixel fillColor = this.options.PadColor.ToPixel<TPixel>();
             bool shouldFill = (this.options.Mode == ResizeMode.BoxPad || this.options.Mode == ResizeMode.Pad)
                               && this.options.PadColor != default;
-            TPixel fillColor = this.options.PadColor.ToPixel<TPixel>();
 
             // Handle resize dimensions identical to the original
             if (source.Width == destination.Width

src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeWorker.cs

@@ -80,14 +80,14 @@ public ResizeWorker(
 
             int numberOfWindowBands = ResizeHelper.CalculateResizeWorkerHeightInWindowBands(
                 this.windowBandHeight,
-                targetWorkingRect.Width,
+                destWidth,
                 workingBufferLimitHintInBytes);
 
             this.workerHeight = Math.Min(this.sourceRectangle.Height, numberOfWindowBands * this.windowBandHeight);
 
             this.transposedFirstPassBuffer = configuration.MemoryAllocator.Allocate2D<Vector4>(
                 this.workerHeight,
-                targetWorkingRect.Width,
+                destWidth,
                 preferContiguosImageBuffers: true,
                 options: AllocationOptions.Clean);