Fix negative shift with IFAST FDCT and qual=100
This fixes regression tests with clang 3.7.0

PR: 202763
Obtained from: https://github.com/libjpeg-turbo/libjpeg-turbo/commit/4cfa3f4c39c2e46eca3a65c67411d15e08a3fc70
Submitted upstream in: https://github.com/mozilla/mozjpeg/pull/186
Approved by: maintainer timeout (2 weeks)