in vyper, the behavior for AugAssign is to perform the bounds checks
only before evaluation of the rhs, rather than before-and-after. in
other words, the following code:

```vyper
def poc():
    a: DynArray[uint256, 2] = [1, 2]
    a[1] += a.pop()
```

is equivalent to:

```vyper
def poc():
    a: DynArray[uint256, 2] = [1, 2]
    a[1] += a[len(a) - 1]
    a.pop()
```

rather than:

```vyper
def poc():
    a: DynArray[uint256, 2] = [1, 2]
    s: uint256 = a[1]
    t: uint256 = a.pop()
    a[1] = s + t  # reverts due to oob access
```

this commit blocks the potentially missing bounds check by panicking
when there is a potential write on the rhs of an AugAssign which could
change the length on the lhs.

references:
- https://github.com/vyperlang/vyper/security/advisories/GHSA-4w26-8p97-f4jp