Equivalent bitrate calculation is broken for <20ms frame sizes

This leads to negative bitrates at smaller frame sizes, which does bad things like making all bands available for intensity stereo coding.

So, for example, encoding with these parameters:

opusenc --hard-cbr --bitrate 192 --framesize 2.5

Yields output that, when mono downmixed, sounds completely terrible, which definitely shouldn't happen at those bitrates (phase inversion intensity stereo should not be used).

Fix: 0001-celt_encoder-fix-equivalent-bitrate-calculation-for-.patch