Fix a bug with a crash when slab alloc arena is full.

When the slab alloc arena is fully used, and we try to
truncate a space, we could crash, since

a) we never check return value of salloc()
b) we used to salloc() an iterator for truncate(), and
it would return NULL, and we would try to access it without a check.

The fix is:

- to throw exceptions directly from salloc()
- to not use salloc() for iterators, since we still should
be able to truncate a namespace when the slab alloc arena is
full.