Fix "left shift of negative value" bug
Fix "left shift of negative value" bug

The values in question are supposed to be able to be negative.

BUG=chromium:653448

Review-Url: https://codereview.webrtc.org/2387333005
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14573}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc