Merge #607: Use size_t shifts when computing a size_t

Merge #607: Use size_t shifts when computing a size_t

e6d01e9 Use size_t shifts when computing a size_t (Pieter Wuille)

Pull request description:

  This was detected by compiling with MSVC; it triggers warning C4334.

  I don't think this is necessary, as we know the maximum shift is a very small integer, but this makes the code more obviously correct.

Tree-SHA512: 3c0cf412c75b4361d01e78bf13fe81c3f28b82abd40b0706285cc691124381cb1ff1f1c3512420250180b7612a471ce48357b282b1e34a08f5359e58af25e198