fix double free resulting from char_p_to_string

[ccbrown]

To quote the docs for CString::from_raw:

This should only ever be called with a pointer that was earlier obtained by calling CString::into_raw. Other usage (e.g., trying to take ownership of a string that was allocated by foreign code) is likely to lead to undefined behavior or allocator corruption.

The added test previously would die when the returned string was dropped with:

malloc: *** error for object 0x137952b14: pointer being freed was not allocated

[nbigaouette]

Thanks!!