fix: 修复判断条件和内存释放问题。

Author: CYJB

Runtime/Text/SourceReader/PartialSourceReader.cs

@@ -407,6 +407,7 @@ protected override void FreeInternal(int index)
 			first = first.Next;
 			buffer.Prev = buffer;
 			buffer.Next = buffer;
+			buffer.Text = string.Empty;
 			first.Prev = first;
 		}
 	}

Runtime/Text/SourceReader/SourceReader.cs

@@ -65,7 +65,7 @@ public static SourceReader Create(string source, int start, int length)
 		{
 			throw CommonExceptions.ArgumentNegative(start);
 		}
-		if (start + length < 0 || start + length >= source.Length)
+		if (start + length < 0 || start + length > source.Length)
 		{
 			throw CommonExceptions.ArgumentNegative(length);
 		}