Fix for ltv crashes and changes for mld improvements

[x] Add fix for ltv crash ./IVAS_dec STEREO 48 bit_error ltv48_STEREO.wav_stereo_32000_48-48_DTX_FER5.tst. Update scale in fd_cng_decfx.c
[x] High MLD Fix - [stv-Multi-channel 5_1 at 24.4 kbps, 48kHz in, 48kHz out, STEREO out]
[x] Fix for 10dB_neg input ltv crash: Test case - ltv-stereo at 32 kbps, 48kHz in, 32kHz out, random FER at 5%
[x] Removed saturation for synthFB buffer
[x] Improved precision for few buffers in MC path