Fix for Termination

Fix for a bug in termination analysis: 
The decreasing parameter in *NOW* checked to be non-negative.

Only 1 failing test: `/benchmarks/text-0.11.2.3/Data/Text/Encoding.hs`: 

```228 : start size ...```
cannot be proven to terminate as there is a call
```
241: let newSize = size * 2
245: start newSize
```