Check if TBrowser/TCanvas has been created on the heap before to call the operator delete

Add also a TBrowser::Destructor() method, to properly clean it up when not calling its destructor.
This fixes the issue reported on the forum: http://root.cern.ch/phpBB3/viewtopic.php?f=3&t=18151&sid=f91b9323579d42d4ccd2dbdd7da05242