return alpha array to array pool

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -529,19 +529,20 @@ namespace ImageSharp.Formats
                 }
 
                 this.WriteChunk(stream, PngChunkTypes.Palette, colorTable, 0, colorTableLength);
+
+                // Write the transparency data
+                if (anyAlpha)
+                {
+                    this.WriteChunk(stream, PngChunkTypes.PaletteAlpha, alphaTable, 0, pixelCount);
+                }
             }
             finally
             {
                 ArrayPool<byte>.Shared.Return(colorTable);
+                ArrayPool<byte>.Shared.Return(alphaTable);
                 ArrayPool<byte>.Shared.Return(bytes);
             }
 
-            // Write the transparency data
-            if (anyAlpha)
-            {
-                this.WriteChunk(stream, PngChunkTypes.PaletteAlpha, alphaTable, 0, pixelCount);
-            }
-
             return quantized;
         }