Compiler segfaults on generic illegal recursion

The following invalid type crashes the compiler:

``` nimrod
type
  TPearl[T] = tuple
    next: TPearl[T]

var x: TPearl[int]

# Hint: used config file '/home/jbe/Documents/nimrod/config/nimrod.cfg' [Conf]
# Hint: system [Processing]
# Hint: issue [Processing]
# [1]    4412 segmentation fault  nimrod c -r issue
```
