Import patch r48211: remove possible calls to new or malloc while handling TShutdownTimer


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@48213 27541ba8-7e3a-0410-8455-c3a389f83636
Modify the Clear function according to the suggestions done here:
http://root.cern.ch/phpBB3/viewtopic.php?f=3&t=15819&p=68057#p68057

This allow to reuse the TGraph2D after a Clear.


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@48214 27541ba8-7e3a-0410-8455-c3a389f83636