Fixes minor code quality issues in CELT

Reported by Durandal.
diff --git a/celt/mathops.c b/celt/mathops.c
index 3f8c5dc..21a01f5 100644
--- a/celt/mathops.c
+++ b/celt/mathops.c
@@ -164,7 +164,7 @@
       {
          return _celt_cos_pi_2(EXTRACT16(x));
       } else {
-         return NEG32(_celt_cos_pi_2(EXTRACT16(65536-x)));
+         return NEG16(_celt_cos_pi_2(EXTRACT16(65536-x)));
       }
    } else {
       if (x&0x0000ffff)