Fix #369 and update test

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -247,7 +247,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             where TPixel : struct, IPixel<TPixel>
         {
             var color = default(TPixel);
-            var rgba = default(Rgba32);
+            var rgba = new Rgba32(0, 0, 0, 255);
 
             using (var buffer = Buffer2D<byte>.CreateClean(width, height))
             {

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -13,7 +13,7 @@ namespace SixLabors.ImageSharp.Tests
     public class BmpDecoderTests : FileTestBase
     {
         [Theory]
-        [WithFileCollection(nameof(AllBmpFiles), PixelTypes.Rgb24)]
+        [WithFileCollection(nameof(AllBmpFiles), PixelTypes.Rgba32)]
         public void DecodeBmp<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : struct, IPixel<TPixel>
         {