[regression] Encoder crash for McMASA 7.1+4 16kHz input at 64kbps in convertToBwMS_fx()
# Basic info
<!--- Add commit SHA used to reproduce -->
- Float reference:
- Encoder (float):
- Decoder (float):
- Fixed point:
- Encoder (fixed): 99e792d19719b5ee09c9e7fa02cfae2ce77ff528
- Decoder (fixed):
# Bug description
The fixed point encoder crashes with an assert:
```
Assertion failed: (0), function L_add, file basop32.c, line 1661.
```
# Ways to reproduce
Box folder: ...\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\issues\issue-1505
<!-- Commandline or script -->
```bash
./IVAS_cod_fx -mc 7_1_4 -max_band WB 64000 16 bm3aa1s05_16.wav bit
# this crash also occurs for 5.1+4 am3aa1s13_16.wav at 48 kbps
```
{width=451 height=248}
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issue