[foreign] RFR 8218772: Limit struct member pointers to size of the field

[Jorn Vernee]

Hi,

I found a bug where it was possible to overwrite trailing fields of a 
struct by writing an oversized array to a previous array field (see 
bug). Overwriting is also possible in other cases by forcing an 
oversized write to a struct field. This can be fixed (relatively easily) 
by limiting the size of memory regions of pointers to struct members to 
the size of their fields.

Please review the following.

Bug: https://bugs.openjdk.java.net/browse/JDK-8218772
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218772/webrev.00/

Thanks,
Jorn