From Philippe:

When copying an histogram into another one, one likely side effect is a
change in the name of the histogram and hence in its hash value.
Before making this change we need to make sure that the histogram has
been removed from its directory (or we will never be able to remove it
and hence could have a dangling pointer!).
This is related to the bug report at
 http://root.cern.ch/phpBB2/viewtopic.php?t=1655


git-svn-id: http://root.cern.ch/svn/root/trunk@11173 27541ba8-7e3a-0410-8455-c3a389f83636