stereo to EXT sets MC renderer

When decodig the IVAS stereo format and using EXT output configuration, the MC renderer is wrongly set. Though the output synthesis is not affected, there are two issues:

• st_ivas->hLsSetUpConversion handle is unnecessary allocated which wastes some static RAM
• when running the decoder and DEBUGGING is activated, the decoder returns the following error:

Internal error: Error: illegal output configuration

In function: audioCfg2channels(), c:\myprojects_ivas\git_3gpp\ivas-codec\lib_rend\ivas_output_init.c:115

Internal fatal error: The conversion matrix between these formats is not defined!

In function: get_ls_conversion_matrix(), ..\lib_dec\ivas_out_setup_conversion.c:296

A fix is straightforward.