Fix a UBSan error in vp9_rc_update_framerate()

Fix a UBSan error in the calculation of rc->min_frame_bandwidth in
vp9_rc_update_framerate().

A follow-up to
https://chromium-review.googlesource.com/c/webm/libvpx/+/4944271.
Similar to the libaom CL
https://aomedia-review.googlesource.com/c/aom/+/190462.

Bug: aomedia:3509
Change-Id: I36168a6d00cd81e60ae19a7d74c21f2e6c2f0caf
(cherry picked from commit 1f65facb63c8227156b5a276e506bb44a7c10608)
2 files changed