capnp: list accessors do not check for element size

[zombiezen]

While lists are being bounds-checked, individual elements are not. If a malicious input contains a valid in-bounds list, but the primitive element being accessed is not in bounds (e.g. the last 64-bit value of a list of 32-bit elements at the end of a region is requested), then the read panics.

This bug was found through use of go-fuzz.