Add missing initialization in TFormula copy constructor.
Add missing initialization in TFormula copy constructor.

Since it calls TFormula::Copy which test on the value of the newly constructed fMethod pointer value,
we must initialized that value.  The other members are simply assign to and thus are fine.

This was seen as a crash when trying the ROOT master with CMSSW.

olifre/root,olifre/root,olifre/root,olifre/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root