Bugfix for histogram scaling function in NetEq's DelayManager.

If the previous value of the histogram is unknown, no scaling should be performed. Without this check a crash would occur. This issue was introduced in https://webrtc-review.googlesource.com/c/src/+/8101, and can only be triggered if the corresponding field trial is set.

Bug: webrtc:8381
Change-Id: I6e7cd8e14f6f4cc972fc094f010ecdf5091b2017
Reviewed-on: https://webrtc-review.googlesource.com/12380
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20336}