Fix point stereo decoder path different from floating point for some FEC cases leading to a crash

with d7c6affd and this bitstream P7c4a-max_band_bwidth_cntl.txt_32000_48_eid.bfi.zip

the fixed point decoder crash at frame 2538. The crash happens in a shl(), but going step by step in the code reveal that some parameters are not insync between the float and fixed during FEC.

so far, at least st->last_good is different.

The ticket will be updated whenever new information is found