NetEq: Changing checked_cast to saturated_cast
NetEq: Changing checked_cast to saturated_cast

The cast involves packet_len_samp, which is derived from the timestamps
and sequence numbers of incoming packets. Being values from the outside,
these should be treated as if any value is possible, making a
checked_cast unsuitable. Changing instead to saturated_cast to avoid
overflow with out-of-bounds values.

Review-Url: https://codereview.webrtc.org/2243403007
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13815}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 38d840c35a85996b9b50294becc2239e8123f67e

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