avformat/utils: Prevent undefined shift with wrap_bits > 64.

2LL << (wrap_bits=64 - 1) does not fit in int64_t; change the
code to use a uint64_t (2ULL) and add an av_assert2() to
ensure wrap_bits <= 64.

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
