Crash in MDCT core decoder for bitrate switching

with 4e186fa8 and using the bitstream in #829 (closed), the decode crash when switching from 24.4 kbps to 64 kbps.

The crash happens at frame 60, in the function ivas_mdct_core_reconstruct_fx().

There at line 2298 you have :

        e_tmp = add( e_nrgL, e_nrgR );

where both e_nrgR and e_nrgL have undefined values. They are both derived from e_sig[0] which is derived from q_syn and q_syn is initialed with st->hTcxDec->q_old_synth at line 2049 which seems to be the source of the problem as it has an un-initialized value (-12851).

My guess is that st->hTcxDec->q_old_synth should be set to a proper value during one of the switching function