Crash in McMASA to custom LS output

The MC format, McMASA mode, is not currently tested for custom LS output.

Moreover, when the number of loudspeakers is less than 4, the decoder crashes in ivas_dirac_dec_render_sf().

An example command line is:

ivas_cod.exe -mc 7_1_4 96000 48 stv714MC48c.wav bit 
ivas_dec.exe ls_setup_3ch.txt 48 bit syn.dec

where ls_setup_3ch.txt

Once the crash is fixed, a self-test condition should be added to cover the automatic testing of McMASA to custom LS output, e.g. using the example command line above.