[TTreeProcessorMT] Breaks without EnableImplicitMT on

TTreeProcessorMT breaks without EnableImplicitMT called before. The problem is, that we access GetImplicitMTPoolSize before creating the pool via TThreadExecutor and then we divide eventually by zero.

We go [0] into MakeClusters, calling [1] GetImplicitMTPoolSize (returning zero, because uninitialized) and then fail shortly after [2] when we divide by zero.

[0] https://github.com/root-project/root/blob/master/tree/treeplayer/src/TTreeProcessorMT.cxx#L500
[1] https://github.com/root-project/root/blob/master/tree/treeplayer/src/TTreeProcessorMT.cxx#L218
[2] https://github.com/root-project/root/blob/master/tree/treeplayer/src/TTreeProcessorMT.cxx#L224

The solution would be creating the pool manager first, which sets the pool size before.