Fixes Opus-custom preemphasis

Also adds denorm avoidance for non-48 kHz rates
diff --git a/celt/celt_encoder.c b/celt/celt_encoder.c
index 1ceb75e..ffff077 100644
--- a/celt/celt_encoder.c
+++ b/celt/celt_encoder.c
@@ -508,7 +508,7 @@
       opus_val16 coef2 = coef[2];
       for (i=0;i<N;i++)
       {
-         opus_val16 x, tmp;
+         celt_sig x, tmp;
          x = inp[i];
          /* Apply pre-emphasis */
          tmp = MULT16_16(coef2, x);