exec() crashes Rust Python when taking large expression as argument

When we set the argument of exec() as a complex expression, Rust Python interpreter crashes Please check the following example. This example triggers segmentation fault on Rust Python 0.1.2 interpreter.

test.py
```
mylist = []
n = 10000
print(exec("mylist"+"+mylist"*n))
```

Actual output:

> Segmentation fault (core dumped)

System:

> Ubuntu 16.04
> Rust Python 0.1.2 interpreter