[regression] Encoder crash for MDCT Stereo at 80 kbps and above in stereo_mdct_core_enc_fx()
# Basic info
<!--- Add commit SHA used to reproduce -->
- Float reference:
- Encoder (float):
- Decoder (float):
- Fixed point:
- Encoder (fixed): d74de338b9d39268bf1db19b2c5792ca359ab02b
- Decoder (fixed):
# Bug description
The fixed point encoder crashes with an assert:
```
Assertion failed: (0), function L_sub, file basop32.c, line 1742.
```
{width=408 height=182}
# Ways to reproduce
Box folder: ...\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\issues\issue-1434
<!-- Commandline or script -->
```bash
./IVAS_cod_fx -stereo -max_band FB 80000 48 dp03a4s04_48.-10dB.wav bit
# this crash also occurs for 96, 128, 160, 192 and 256 kbps
```
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issue