bug in solve_mod -- variable sorting

<div id="comment:0"></div>


```
Carlo Hamalainen: 
>  OK, but in solve_mod() perhaps the line
>  
>     vars.sort()
>  
>  should be
>  
>     vars.sort(cmp)
>  
>  so that the variables are actually sorted?
>  

Yes, *that* is certainly a bug!

William
```

Component: **calculus**

_Issue created by migration from https://trac.sagemath.org/ticket/3135_

