UBSAN: assignments between unsigned and signed int change values in TonalMDCTConceal_SaveFreqSignal

In tonalMDCTconcealment.c:129, nOldSamples is defined as int16_t, but gets assigned to from unsigned int variables and later its value is used to set an unsigned integer variable. UBSAN reports that this can change values there. This is only reported for the first frame and thus probably not an issue in general. Changing the variable type to uint16_t should be BE and silences the error.