Use-of-uninitialized value for rate switching scenarios and PLC with MDCT_STEREO_PLC_FADE_2_BG_NOISE

Basic info

• Commit SHA: 3d656b64
• Platform: Linux

Bug description

Clang msan error when the last frame of a DFT-Stereo mode is lost and is concealed with TCX FD concealment and the next good frame is in a bitrate with MDT-Stereo. See https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/jobs/34979. Solution is to synch st->hTcxDec->L_frameTcx for both channels after allocating a second core coder since it is uninitialized int these scenarios, but needed in ivas_core_dec.c:283++.