PLC discontinuities in DFT Stereo

In stereo_dft_dec_core_switching() a wrong path is taken if the frame after a PLC frame with TCX concealment is ACELP. In this case a TCX->ACELP transition should be triggered. However, last_core is overwritten with last_core_bfi which is ACELP_CORE also for TCX concealment. Therefore, the regular non-transition ACELP path is currently taken.

This can lead to (sometimes severe) discontinuities in the lowband (see spectrogram below).