OpenVM allows the byte decomposition of pc in AUIPC chip to overflow

The fix to https://cantina.xyz/code/c486d600-bed0-4fc6-aed1-de759fd29fa2/findings/21 has a typo that still results in the highest limb of pc being range checked to 8-bits instead of 6-bits.

In the AIR, we do https://github.com/openvm-org/openvm/blob/0f94c8a3dfa7536c1231465d1bdee5fc607a5993/extensions/rv32im/circuit/src/auipc/core.rs#L135

        for (i, limb) in pc_limbs.iter().skip(1).enumerate() {
            if i == pc_limbs.len() - 1 {

It should be

        for (i, limb) in pc_limbs.iter().enumerate().skip(1) {

Right now the if statement is never triggered because the enumeration gives i=0,1,2 when we instead want i=1,2,3. What this means is that pc_limbs[3] is range checked to 8-bits instead of 6-bits.

This leads to a vulnerability where the pc_limbs decomposition differs from the true pc, which means a malicious prover can make the destination register take a different value than the AUIPC instruction dictates, by making the decomposition overflow the BabyBear field.