decode_vmslice is misleading

[jayz22]

The decode_vmslice function is misleading. It doesn't decode anything, it is merely cloning out the VM (or error if not a in contract), and return it with whatever pos and len values that have been passed in, even if those values do not correspond to valid linear memory range. It does not perform any checks.
It is being used wrong in several places. e.g. in map_new_from_linear_memory, the len is actually the number of slices (8-bytes pos|len code), not the number of bytes.