Fixes regression introduced by r5016 (exception produced when connecting to 8bpp server)

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@5017 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/java/com/tigervnc/rfb/PixelBuffer.java b/java/com/tigervnc/rfb/PixelBuffer.java
index 4627318..d7380b7 100644
--- a/java/com/tigervnc/rfb/PixelBuffer.java
+++ b/java/com/tigervnc/rfb/PixelBuffer.java
@@ -41,8 +41,11 @@
     case  6: 
       // Fall-through to depth 8
     case  8: 
-      if (cm != null && cm instanceof IndexColorModel)
+      if (!pf.trueColour) {
+        if (cm == null)
+          cm = new IndexColorModel(8, 256, new byte[256], new byte[256], new byte[256]);
         break;
+      }
       int rmask = pf.redMax << pf.redShift;
       int gmask = pf.greenMax << pf.greenShift;
       int bmask = pf.blueMax << pf.blueShift;