Gaurd agaisnt null data when adding a new ImageFrame from data

src/ImageSharp/ImageFrameCollection.cs

@@ -77,6 +77,8 @@ namespace SixLabors.ImageSharp
         /// <inheritdoc/>
         public ImageFrame<TPixel> AddFrame(TPixel[] data)
         {
+            Guard.NotNull(data, nameof(data));
+
             var frame = ImageFrame.LoadPixelData(
                 this.parent.GetMemoryManager(),
                 new Span<TPixel>(data),

tests/ImageSharp.Tests/Image/ImageFramesCollectionTests.cs

@@ -39,7 +39,6 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void AddNewFrame_Frame_FramesNotBeNull()
         {
-
             ArgumentNullException ex = Assert.Throws<ArgumentNullException>(() =>
             {
                 this.collection.AddFrame((ImageFrame<Rgba32>)null);
@@ -49,12 +48,13 @@ namespace SixLabors.ImageSharp.Tests
         }
 
         [Fact]
-        public void AddNewFrame_PixelBuffer_FramesNotBeNull()
+        public void AddNewFrame_PixelBuffer_DataMustNotBeNull()
         {
+            Rgba32[] data = null;
 
             ArgumentNullException ex = Assert.Throws<ArgumentNullException>(() =>
             {
-                this.collection.AddFrame((Rgba32[])null);
+                this.collection.AddFrame(data);
             });
 
             Assert.StartsWith("Value cannot be null.", ex.Message);
@@ -63,7 +63,6 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void AddNewFrame_PixelBuffer_BufferIncorrectSize()
         {
-
             ArgumentOutOfRangeException ex = Assert.Throws<ArgumentOutOfRangeException>(() =>
             {
                 this.collection.AddFrame(new Rgba32[0]);
@@ -75,7 +74,6 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void InsertNewFrame_FramesMustHaveSameSize()
         {
-
             ArgumentException ex = Assert.Throws<ArgumentException>(() =>
             {
                 this.collection.InsertFrame(1, new ImageFrame<Rgba32>(Configuration.Default.MemoryManager, 1, 1));
@@ -87,7 +85,6 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void InsertNewFrame_FramesNotBeNull()
         {
-
             ArgumentNullException ex = Assert.Throws<ArgumentNullException>(() =>
             {
                 this.collection.InsertFrame(1, null);
@@ -99,7 +96,6 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void Constructor_FramesMustHaveSameSize()
         {
-
             ArgumentException ex = Assert.Throws<ArgumentException>(() =>
             {
                 var collection = new ImageFrameCollection<Rgba32>(this.image, new[] {