Fix by Philippe to a problem reported by Maarten

TFormula::TFormula(TFormula const &) was indirecly accessing uninitialized memory.
This is fixed by initializing the value in it.

TF1::Copy was leaking memory (not deleting existing arrays), once this was fixed,
this required to add the proper initialization in TF1::TF1(TF1 const &).


git-svn-id: http://root.cern.ch/svn/root/trunk@7170 27541ba8-7e3a-0410-8455-c3a389f83636