cosmwasm_vm::memory::read_region panics on ptr == 0

[reuvenpo]

This is an issue because the ptr is supplied in the contract when calling the function from wasm imports.
This is not disastrous because Rust panics are caught just before returning to Go, but we can handle this more gracefully.
For example, we have the maybe_read_region function, that returns a None when the ptr is 0. We can use this function instead, and then convert the None case to a variant of VmError. This will then mesh well with all the other error handling in the VM.