HAVE_FAST_UNALIGNED being true does not imply that
one can simply read from any pointer via *(long*).
It is undefined behaviour in case the pointer is not
sufficiently aligned; and even if it is, it is (likely)
a violation of the effective-type rules. Fix both
of these by using the appropriate AV_[RW]N macros.

Also, the current code used sizeof(long) as if this
were the CPU's native arithmetic size, but this is
not true on 64bit Windows. This has been fixed, too.

This affected huffyuv FATE-tests.
Tested-by: Sean McGovern <gseanmcg@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>