Fix for left shift of potentially negative value.
Fix for left shift of potentially negative value.

Left shifting of negative integers is undefined behavior, and should be prevented. This CL fixes one such instance in the Levinson Durbin function.

BUG=chromium:681377

Review-Url: https://codereview.webrtc.org/2680973005
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16507}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a48e1b6bc3d26dfc70748f05112617e8fddb32fa

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