This is a tentative fix for a bug found in fuzzing where the encoder
switched from mono to stereo while in the process of changing bandwidth.
The result was that the newly added side would use the new sampling
rate, while the mid hadn't switched yet, causing an encoder/decoder
mismatch. The fix is that the side rate selection gets overridden
to use the mid rate.
The bug would occur when compiling with fuzzing enabled and using:
./test_opus 0 48000 2 24000 input.sw output.sw