Avoids pre-echo in hybrid mode caused by noise being injected in the first band

This could happen when we had more than 32 bits on the first hybrid band with
a transient just in the middle of the frame. The band would be split and the
first half of the frame could end up with non-zero energy, but not enough
bits for a pulse. Because it's the first band, no folding would be possible.
This would cause noise to be injected for the entire duration of the first
half and that noise should then get folded to higher bands.