program: safe deserialize instruction

[buffalojoec]

Problem

Similar to solana-program/config#16, the Address Lookup Table program imposes a limit on the provided instruction buffer length. However, the ExtendLookupTable instruction uses a vector of pubkeys, resulting in a vector length stored in the first 8 bytes of the buffer after the discriminator.

If that vector length is very large, even if the rest of the buffer is invalid, the allocator in the BPF version will attempt to allocate a large vector and exhaust the heap.

Summary of Changes

Add a safe_deserialize_instruction helper to check the vector length before attempting deserialization.

[joncinque]

This makes sense to me! Just a few little bits for maintenance

[joncinque]

Looks great, thanks!