Moved vtable definition in ab_tag_create

Moved the creation of the vtable to earlier in ab_tag_create. When
failures occured after memory was allocated, but before the vtable
was setup, the destroy methods were not called and memory was not
reclaimed. This led to multiple memory leaks.
Merge pull request #2 from ericorange/move_vtable

Moved vtable definition in ab_tag_create