Extend GetInvInt to support values up to 54

For sample rates >= 64 kHz with short blocks, the largest sfb
width is 36, which after grouping can be multiplied with 3,
ending up as 108. GetInvInt needs to be able to support
the largest grouped sfb width, divided by two (3 * 36 / 2 = 54).

This fixes VBR encoding with sample rates >= 64 kHz.