hoa_dec_mtx handle in SBA decoder

The hoa_dec_mtx handle is used only partially at some SBA format decoder operating points. E.g. for HOA3 at 512 kbps, stereo or HOA3 output, the consumption is as follows:

• ivas_sba_get_hoa_d 119 m:16 * 16 * sizeof( float ) - 13% 1x256 256

*hoa_dec_mtx = (float *) count_malloc( SBA_NHARM_HOA3 * MAX_OUTPUT_CHANNELS * sizeof( float ) )

The same was observed when decoding to the HOA3 output.

This is a follow-up of #123 (closed) and the issue can be seen when MEM_COUNT_DETAILS is activated.