Implement IEquatable<Block8x8F> and check when summing.

src/ImageSharp/Formats/Jpeg/Components/Block8x8F.cs

@@ -14,7 +14,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
     /// <summary>
     /// Represents a Jpeg block with <see cref="float"/> coefficients.
     /// </summary>
-    internal partial struct Block8x8F
+    internal partial struct Block8x8F : IEquatable<Block8x8F>
     {
         /// <summary>
         /// A number of scalar coefficients in a <see cref="Block8x8F"/>
@@ -538,6 +538,27 @@ public void LoadFromInt16ExtendedAvx2(ref Block8x8 source)
             Unsafe.Add(ref dRef, 7) = bottom;
         }
 
+        /// <inheritdoc />
+        public bool Equals(Block8x8F other)
+        {
+            return this.V0L == other.V0L
+            && this.V0R == other.V0R
+            && this.V1L == other.V1L
+            && this.V1R == other.V1R
+            && this.V2L == other.V2L
+            && this.V2R == other.V2R
+            && this.V3L == other.V3L
+            && this.V3R == other.V3R
+            && this.V4L == other.V4L
+            && this.V4R == other.V4R
+            && this.V5L == other.V5L
+            && this.V5R == other.V5R
+            && this.V6L == other.V6L
+            && this.V6R == other.V6R
+            && this.V7L == other.V7L
+            && this.V7R == other.V7R;
+        }
+
         /// <inheritdoc />
         public override string ToString()
         {

src/ImageSharp/Formats/Jpeg/Components/Decoder/QualityEvaluator.cs

@@ -84,9 +84,13 @@ public static int EstimateQuality(Block8x8F[] quantizationTables)
             for (int i = 0; i < quantizationTables.Length; i++)
             {
                 ref Block8x8F qTable = ref quantizationTables[i];
-                for (int j = 0; j < Block8x8F.Size; j++)
+
+                if (!qTable.Equals(default))
                 {
-                    sum += qTable[j];
+                    for (int j = 0; j < Block8x8F.Size; j++)
+                    {
+                        sum += qTable[j];
+                    }
                 }
             }
 

tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.MetaData.cs

@@ -52,8 +52,9 @@ public partial class JpegDecoderTests
         public static readonly TheoryData<string, int> QualityFiles =
         new TheoryData<string, int>
         {
-            { TestImages.Jpeg.Baseline.Calliphora, 80},
-            { TestImages.Jpeg.Progressive.Fb, 75 }
+            { TestImages.Jpeg.Baseline.Calliphora, 80 },
+            { TestImages.Jpeg.Progressive.Fb, 75 },
+            { TestImages.Jpeg.Issues.IncorrectQuality845, 99 }
         };
 
         [Theory]

tests/ImageSharp.Tests/TestImages.cs

@@ -172,6 +172,7 @@ public static class Issues
                 public const string OrderedInterleavedProgressive723C = "Jpg/issues/Issue723-Ordered-Interleaved-Progressive-C.jpg";
                 public const string ExifGetString750Transform = "Jpg/issues/issue750-exif-tranform.jpg";
                 public const string ExifGetString750Load = "Jpg/issues/issue750-exif-load.jpg";
+                public const string IncorrectQuality845 = "Jpg/issues/Issue845-Incorrect-Quality99.jpg";
 
                 public static class Fuzz
                 {