Running the NanoReaderArray on nonexisting branches throws silently

[l-cadamuro]

The NanoReaderArray does not attach the ttrereader pointer if the branch does not exist.
In 2017 data the b tag correction branch changed of name and this causes a silent crash when this value is accessed.
The dereferencing of the ttrereader ptr in the class can be simply extended with a validity check to throw in case the pointer is not valid.