Fix Undefined-shift in ubidi_getRuns

Upstream PR: https://github.com/unicode-org/icu/pull/663

Bug: chromium:966377
Change-Id: Icde4128f0c333e6c1cf79704789f893b9a5f3780
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/1637963


Reviewed-by: Jungshik Shin <jshin@chromium.org>