Out of bounds memory load, store - PREVAIL assumes context is non-null but uBPF sets context to null

[tobashi]

We generated programs which result in an out of bounds memory load and store but is verified by PREVAIL.

mov64 r0, 0x0
arsh64 r0, r5
ldxw r3, [r1+0x1]
mov64 r4, r1
exit

I think we concluded that this is an issue with uBPF rather than PREVAIL, where ctx is null
vbpf/ebpf-verifier#492