BASOP asserts in MC 5.1 fx encoder for selection test inputs

Basic info

• Git SHA: ecba0c59

Bug description

Selection test inputs for 5.1 MC trigger asserts in the fixed-point encoder:

Frames processed:       255     Assertion failed: (0), function L_shl, file basop32.c, line 2582.

Ways to reproduce

for bitrate in 13200 16400 24400 32000; do                                                                                                                                                                                                                                                           
    ./IVAS_cod_fx -mc 5_1 -max_band FB $bitrate 48 am2aa1s02_48.wav bit
done