BASOP assert in 48kbps stereo decoding due to left shift of negative value
Using item 26 of SQAM test set resampled to 48kbps,
```
./IVAS_cod_flt -stereo 48000 48 26_48kHz.wav bit
./IVAS_dec_fx stereo 48 bit out.wav
```
results in
```
Assertion failed: (0), function L_shl, file lib_com/basop32.c, line 2438.
```
in frame 1500. Caller context is `tcx_mdct_fx.c:305:
``` c
FOR( i = 0; i < R2; i++ )
{
y[l + m + R2 + i] = round_fx( L_shl( Mpy_32_16_1( tmp_buf[L2 + i], fac ), s ) ); /* fold out right end of DCT */
}
```
with i=6, fac=20066 and L2=30.
issue