## BarrelList has three index-boundary bugs: out-of-bounds `__getitem__`, wrong `insert` position, and `pop()` crash on drained tail

### Description

After a `BarrelList` has been split into multiple internal sublists (which happens after mutating operations trigger `_balance_list`), several boundary conditions are handled incorrectly.

Here's a helper to get a multi-sublist `BarrelList`:

```python
from boltons.listutils import BarrelList

def make_bl(n=30000):
    bl = BarrelList(range(n))
    bl.pop(0)
    bl.insert(0, 0)
    # now bl.lists has more than one sublist
    return bl
```

**Bug 1: `bl[len(bl)]` does not raise `IndexError`**

Accessing an index equal to the length of the list should raise `IndexError` (matching standard Python list behavior), but instead it silently returns a value from somewhere in the middle of the list:

```python
bl = make_bl()
bl[len(bl)]  # should raise IndexError, but returns an integer
```

**Bug 2: `insert` at or past the end doesn't append**

Python's `list.insert(i, x)` clamps out-of-bounds indices: `insert(len(lst), x)` appends to the end. `BarrelList.insert` does not do this — it inserts at the wrong position:

```python
bl = make_bl()
bl.insert(len(bl), 'end')
print(bl[-1])  # prints 29999, expected 'end'
```

**Bug 3: `pop()` raises `IndexError` after tail sublist is drained**

If you remove all elements from the last internal sublist via indexed pops, the tail sublist becomes empty. A subsequent `bl.pop()` (no-arg, should pop the last element) then crashes with `IndexError: pop from empty list` even though the `BarrelList` still has elements:

```python
bl = make_bl()
for _ in range(len(bl.lists[-1])):
    bl.pop(len(bl) - 1)  # drain the tail sublist
bl.pop()  # IndexError: pop from empty list
```

### Expected behavior

- `bl[len(bl)]` raises `IndexError`
- `bl.insert(len(bl), x)` appends `x` to the end; `bl[-1] == x`
- `bl.insert(len(bl) + 100, x)` also appends; `bl.insert(-len(bl) - 100, x)` prepends
- `bl.pop()` returns the last element even when the tail internal sublist has been emptied by prior indexed removals

### Actual behavior

- `bl[len(bl)]` returns a mid-list element instead of raising `IndexError`
- `bl.insert(len(bl), 'end')` inserts somewhere in the middle; `bl[-1]` is still `29999`
- `bl.pop()` raises `IndexError: pop from empty list` on a non-empty `BarrelList`

Work in `/workspace`. Submit your fix in the existing Python source files under `boltons`. Preserve the other public behavior. The environment is offline; dependencies are preinstalled. Grading runs the repository's test suite in a fresh environment, using your submitted source files.