When using Unpacker as an iterator, after each yield, the internal
buffer (_fb_buffer) was compacted by reallocation (done by _fb_consume).
When dealing with a lot of small objects, this is very ineffecient.
Thus in commit 7eb371f8278941fb2323e0c2333ed89c88ab822b the pure python
fallback only reallocated the complete buffer when the iteration stops.
When halfway there happens to be data missing in the buffer, we rollback
the buffer to the state before this failed call, and raise an OutOfData.
This rollback, done by _fb_rollback, did not consider the possibility
that the buffer was *not* reallocated. This commit corrects that.