Unchecked calls to getField(), getSubField(), etc.

This module needs to be audited for unchecked calls to methods which can return NULL.  This is especially problematic in the ```isCompatible()``` methods which are intended to be used on suspect type definitions.

https://github.com/epics-base/normativeTypesCPP/blob/5e151c3b7a23ec4f04332b587349c6ae542d4dd2/src/ntndarray.cpp#L352

cf. areaDetector/ADCore#296