Fix loading SVG table with 0 entries

src/SixLabors.Fonts/Tables/General/Svg/SvgTable.cs

@@ -74,8 +74,8 @@ private SvgTable(byte[] tableData, uint svgDocIndexOffset, uint tableBaseOffset,
     /// Loads the SVG table from the specified binary reader.
     /// </summary>
     /// <param name="reader">The binary reader positioned at the start of the SVG table.</param>
-    /// <returns>The <see cref="SvgTable"/>.</returns>
-    public static SvgTable Load(BigEndianBinaryReader reader)
+    /// <returns>The <see cref="SvgTable"/>, or <see langword="null"/> if the table is not valid in the font.</returns>
+    public static SvgTable? Load(BigEndianBinaryReader reader)
     {
         // HEADER
         // | Type     | Name              | Description                                               |
@@ -99,6 +99,12 @@ public static SvgTable Load(BigEndianBinaryReader reader)
         // | Entry[numEntries] | entries    | Array of SVG Document Index Entries(sorted by startGlyphID).|
         reader.Seek(svgDocIndexOffset, SeekOrigin.Begin);
         ushort numEntries = reader.ReadUInt16();
+        if (numEntries == 0)
+        {
+            // The spec says the number of entries must be non-zero.
+            return null;
+        }
+
         SvgDocumentIndexEntry[] entries = new SvgDocumentIndexEntry[numEntries];
 
         // SVG Document Index Entry

tests/SixLabors.Fonts.Tests/Issues/Issues_534.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.Fonts.Tests.Issues;
+
+public class Issues_534
+{
+    [Fact]
+    public void ShouldLoadFontWithSvgTableHavingZeroEntries()
+    {
+        Font font = TestFonts.GetFont(TestFonts.Issues.Issue534, 12);
+
+        FontRectangle size = TextMeasurer.MeasureBounds("ABCabc123", new TextOptions(font));
+
+        Assert.NotEqual(FontRectangle.Empty, size);
+    }
+}

tests/SixLabors.Fonts.Tests/TestFonts.cs

@@ -414,6 +414,8 @@ public static class Issues
         public static string Issue512 => GetFullPath("Issues/Issue512.otf");
 
         public static string Issue514 => GetFullPath("Issues/Issue514.ttf");
+
+        public static string Issue534 => GetFullPath("Issues/Issue534.ttf");
     }
 
     /// <summary>