Encoder crash for Stereo at 16.4kbps in prep_tbe_exc_fx()
# Basic info
<!--- Add commit SHA used to reproduce-->
- Fixed point:
- Encoder (fixed): 8cdb71ccb83b5cd408fe2ac84b86c27704ea3618
# Bug description
The fixed point encoder crashes with an assert:
```
Assertion failed: (0), function L_shl, file basop32.c, line 2332.
```
{width=410 height=195}
# Ways to reproduce
Box folder: ...\\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\\issues\\issue-1804
<!--Commandline or script-->
```bash
./IVAS_cod -stereo -max_band WB 16400 32 dm1aa1s08_32.10dB.wav bit
```
<!--- Below are labels that will be added but are not shown in description. This is a template to help fill them.
Add further information to the first row and remove and add labels as necessary.-->
issue