[nem-en] conversion of int to uint

[Valient Gough]

Hi, I suspect that I've found a bug (in which case I'll put in a bug
ticket instead).  But I wanted to ask first, in case that I'm just
going about this the wrong way.

I'm having trouble converting from an int to uint.  I have to convert
to a common type because I need to apply a bit-field operation, and
I'm constrained on the different types from external interfaces.

def checkMask( mask, modes )
{

}