fix #45, avoid inconsistent results for incorrectly serialized bitmaps

[ChenHuajun]

fix #45, avoid inconsistent results for incorrectly serialized bitmaps when using roaring_buffer_reader

[ChenHuajun]

Use the following SQL to compare performance and memory usage with and without this patch.

select rb_or_cardinality_agg('\x3a3000009900000000008e0201008e0202...'::roaringbitmap) from generate_series(1,1000000);

Cardinality of the roaringbitmap object is about 100,000

without this patch:
memory: 22680KB
time: 2m27s

with this patch:
memory: 21412KB
time: 2m26s