Fix bug in TFileIO.CheckBOM (TStream overload)

Method was returning True for encodings with zero length BOMs instead of
the False result per documentation.

Fixes #139

Author: delphidabbler

Diff for: Src/UIOUtils.pas

@@ -206,6 +206,8 @@ class function TFileIO.CheckBOM(const Stream: TStream;
   Assert(Assigned(Stream), 'TFileIO.CheckBOM: Stream is nil');
   Assert(Assigned(Encoding), 'TFileIO.CheckBOM: Encoding is nil');
   Preamble := Encoding.GetPreamble;
+  if Length(Preamble) = 0 then
+    Exit(False);
   if Stream.Size < Length(Preamble) then
     Exit(False);
   OldPos := Stream.Position;