Validate Region in get_region

[webmaster128]

When working on AssemblyScript support, I ran in many different situations where wrong Region pointers where communicated to the VM. Since you can basically dereference any 12 bytes of memory to a Region, those situations were hard to debug. In order to error out as soon as possible, we can do some validation on the Region.

The following things can be validated:

• length <= capacity
• offset != 0
• offset + capacity is within the size of the memory