Added sanity check to VCMDecodingState::UsingFlexibleMode to prevent OOB error.
Added sanity check to VCMDecodingState::UsingFlexibleMode to prevent OOB error.

BUG=chromium:667504

Review-Url: https://codereview.webrtc.org/2534883003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15298}

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