Fix pattern matching to avoid IndexOutOfRangeException (#9357)

This fixes a problem introduce late into the file path normalization
pull request.

Author: DustinCampbell

src/Razor/src/Microsoft.AspNetCore.Razor.ProjectEngineHost/Utilities/FilePathNormalizer.cs

@@ -28,7 +28,7 @@ public static string NormalizeDirectory(string? directoryFilePath)
         ReadOnlySpan<char> normalizedSpan = arraySpan.Slice(start, length);
 
         // Add a trailing slash if the normalized span doesn't end in one.
-        if (normalizedSpan[^1] != '/')
+        if (normalizedSpan is not [.., '/'])
         {
             arraySpan[start + length] = '/';
             normalizedSpan = arraySpan.Slice(start, length + 1);
@@ -169,7 +169,7 @@ private static (int start, int length) NormalizeCore(ReadOnlySpan<char> source,
         destination[..charsWritten].Replace('\\', '/');
 
         if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows) &&
-            destination is ['/', not '/', ..])
+            destination is ['/', ..] and not ['/', '/', ..])
         {
             // We've been provided a path that probably looks something like /C:/path/to.
             // So, we adjust the result to skip the leading '/'.

src/Razor/test/Microsoft.AspNetCore.Razor.ProjectEngineHost.Test/FilePathNormalizerTest.cs

@@ -62,6 +62,19 @@ public void NormalizeDirectory_EndsWithoutSlash()
         Assert.Equal("C:/path/to/directory/", normalized);
     }
 
+    [OSSkipConditionFact(new[] { "OSX", "Linux" })]
+    public void NormalizeDirectory_Windows_HandlesSingleSlashDirectory()
+    {
+        // Arrange
+        var directory = @"\";
+
+        // Act
+        var normalized = FilePathNormalizer.NormalizeDirectory(directory);
+
+        // Assert
+        Assert.Equal("/", normalized);
+    }
+
     [Fact]
     public void FilePathsEquivalent_NotEqualPaths_ReturnsFalse()
     {