integer overflow in vtk/lion encoding

[cwsmith]

A 416M element serial mesh causes integer overflows in a few places:

core/lion/lionBase64.cc

Line 136 in e3b5de4

unsigned int index = 0;

core/apf/apfVtk.cc

Line 459 in e3b5de4

unsigned int dataLenBytes = dataLen*sizeof(int);