The shift-amount can be up to 56, and left-shifting 32-bit integers
by values >=32 is undefined behaviour. Therefore, use 64-bit integers
instead. Also slightly rewrite so we only call dav1d_get_bits() once
for the combined more|bits value, and mask the relevant portions
out instead of reading twice. Lastly, move the overflow check out of
the loop (as suggested by @wtc)

Fixes #341.