Fix misaligned word access in redisPopcount().
BITCOUNT fuzzy test with random start/end added.

It was verified in practice that this test is able to stress much more
the implementation by introducing errors that were only trivially to
detect with different offsets but impossible to detect starting always
at zero and counting bits the full length of the string.
Added two more BITCOUNT tests stressing misaligned access.
Merge branch 'unstable' of github.com:/antirez/redis into unstable