Fix undefined left shift of negative value

celt/rate.c

@@ -189,7 +189,7 @@ void compute_pulse_cache(CELTMode *m, int LM)
                   /* Offset the number of qtheta bits by log2(N)/2
                       + QTHETA_OFFSET compared to their "fair share" of
                       total/N */
-                  offset = ((m->logN[j]+((LM0+k)<<BITRES))>>1)-QTHETA_OFFSET;
+                  offset = ((m->logN[j]+(opus_int32)((opus_uint32)(LM0+k)<<BITRES))>>1)-QTHETA_OFFSET;
                   /* The number of qtheta bits we'll allocate if the remainder
                       is to be max_bits.
                      The average measured cost for theta is 0.89701 times qb,