Add missing protection in OptimizeBasket.

Some of the branch (because they have been disabled for
example) may not have any entry.  So we now protect
against this case (actually seen in stress).

This fixes ROOT-5410
Avoid spurrious round-trip through string based interface

Add TMethodCall::Init(TFunction*)
to avoid having to go back to the string based interface and
having to do yet another lookup when we already have selected
the function/method we need and thus have already all the
information.