Decoder crash for ParamMC 5.1 at 48/64/80 kbps decoding to mono in ivas_ls_setup_conversion_fx()
# Basic info
<!--- Add commit SHA used to reproduce-->
- Float reference:
- Encoder (float): 31b60873224ef26efcbfd8a579afc2765a0106a9
- Decoder (float):
- Fixed point:
- Encoder (fixed):
- Decoder (fixed): 339e804c7cf5c345ac74d254ad89732e1a92b399
# Bug description
The fixed point decoder crashes with an assert:
```
Assertion failed: (0), function L_add, file basop32.c, line 1661.
```
{width=388 height=178}
# Ways to reproduce
Box folder: ...\\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\\issues\\issue-1408
<!--Commandline or script-->
```bash
./IVAS_cod_flt_ref -mc 5_1 -max_band FB 48000 48 am2aa1s06_48.10dB.wav bit
./IVAS_dec_fx mono 48 bit out.wav
```
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issue