hIGFEnc handle in SBA

The hIGFEnc handle is allocated but never used at some SBA encoder operating points. E.g. for HOA3 at 512 kbps, the consumption is as follows:

• init_encoder 768 m:sizeof( IGF_ENC_INSTANCE ) - 0% 4x2071 8284

st->hIGFEnc = (IGF_ENC_INSTANCE_HANDLE) count_malloc( sizeof( IGF_ENC_INSTANCE ) )

At the decoder for stereo output, the handle is used only partially:

• init_decoder 680 m:sizeof( IGFDEC_INSTANCE ) - 37% 4x2692 10768

st->hIGFDec = (IGF_DEC_INSTANCE_HANDLE) count_malloc( sizeof( IGFDEC_INSTANCE ) )

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