Backward compatibility patch.

Without this patch, TSelectorDraw can not longer detect parsing failure
and in case of parsing failure would return 0 instead of -1.
(as demonstrated by roottest failures in root/treeformula/array)


git-svn-id: http://root.cern.ch/svn/root/trunk@15710 27541ba8-7e3a-0410-8455-c3a389f83636