fixing inferred latch

The warning can be completely ignored. Quartus has given this message for years. I told Altera about it a long time ago and I guess they haven't done anything about it. The message is given during synthesis. Basically the compiler is looking at idx as an actual piece of logic rather than a simple loop iterator in the language. If you look at the final compiled RTL though, you will find no instance of "idx" anywhere. Just ignore it.