Make type conversion explicit

Change-Id: I53d5a29c1dc1c93535e1e6c6bef34f232feb5e1e
diff --git a/av1/common/av1_txfm.h b/av1/common/av1_txfm.h
index 1f45254..019f761 100644
--- a/av1/common/av1_txfm.h
+++ b/av1/common/av1_txfm.h
@@ -105,7 +105,7 @@
 static INLINE int32_t half_btf(int32_t w0, int32_t in0, int32_t w1, int32_t in1,
                                int bit) {
   int32_t result_32 = (int32_t)clamp64((int64_t)w0 * in0 + (int64_t)w1 * in1,
-                                       INT32_MIN, INT32_MAX);
+                                       (int64_t)INT32_MIN, (int64_t)INT32_MAX);
   return round_shift(result_32, bit);
 }