RFR: allow empty group layout

[Henry Jen]

Hi,

Please review another tweak[1] on DescriptorParser to allow empty group layout, this could happen with empty struct or union, although that’s not valid C and I doubt anyone would actually wrote that code.

However, this is valid in C++ and some C compiler seems to allow it[2], so we should probably be OK with it. This StackOverflow entry[3] has some interesting links regarding empty struct.

Cheers,
Henry

[1] http://cr.openjdk.java.net/~henryjen/panama/foreign/emptyStruct/webrev/
[2] https://gcc.gnu.org/onlinedocs/gcc/Empty-Structures.html
[3] https://stackoverflow.com/questions/3849334/sizeof-empty-structure-is-0-in-c-and-1-in-c-why