Fix memory access masking bug for nonPow2 memories

[ccelio]

I've found that for a NonPow2 sized memory, I was getting garbage values that were causing buffer overruns elsewhere in my Chisel emulator (feeding a mem.read() value into another mem() access).

I'd like a code review to verify my logic on this is correct.

[ccelio]

I'm not familiar with the unit-testing infrastructure - what does it take to add a unit test for non-pow2 memories?

[ucbjrl]

@ccelio we're still using ancient JUnit test style code and the old Tester with chisel2. Take a look at src/test/scala/SeqMemSuite.scala for a simple example.

[aswaterman]

Closed with 1ee157fea9bc5816bce04aadaa554b8970cb8871