RooAddPdf: Avoid UB in checkObservables
RooAddPdf: Avoid UB in checkObservables

A RooAddPdf may have more PDFs than coefficients, in which case
"the coefficient of the last PDF is calculated automatically from
the condition that the sum of all coefficients has to be 1". In
this case, the last call to "_coefList.at(i)" is supposed to return
a nullptr because the index is out of range, and dereferencing it
is undefined behavior which Clang 13 optimizes away, leading to
crashes.

Fixes #9547

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root