Make HcalCholeskyMatrix compatible with new ROOT

ROOT versions before 5.34.20 did not properly store a multi-dimensional
array. Instead, they only stored such arrays as if they were one dimensional
equal to the first dimension. In this case, only the first 4 array elements
were ever stored. With versions 5.34.20 and beyond this was fixed, however
it means the data stored in the conditions database is no longer readable.
Changing HcalCholeskyMatrix to match what ROOT did in the old versions allows
us to read the old data. If a value outside of the stored range is requested,
the code returns the value of 0 which matches the old behavior.