Fix memory corruption bug.

This bug occured when a second telepathy tubes client was connected after
the first one had disconnected and the channel (thus, the screen too)
had been destroyed.

git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/KDE/kdenetwork/krfb/libvncserver@1195288 283d02a7-25f6-0310-bc7c-ecb5cbfe19da

tight.c

@@ -118,10 +118,12 @@ void rfbTightCleanup(rfbScreenInfoPtr screen)
 {
   if(tightBeforeBufSize) {
     free(tightBeforeBuf);
+    tightBeforeBuf = NULL;
     tightBeforeBufSize=0;
   }
   if(tightAfterBufSize) {
     free(tightAfterBuf);
+    tightAfterBuf = NULL;
     tightAfterBufSize=0;
   }
 }