Clamp int64_t first before its int32_t truncation

BUG=aomedia:1960

Change-Id: I66a34953a0c966fcee8a0bf20e47b6b6a879e551
diff --git a/av1/common/av1_txfm.c b/av1/common/av1_txfm.c
index 3694c25..9b2f5f9 100644
--- a/av1/common/av1_txfm.c
+++ b/av1/common/av1_txfm.c
@@ -78,7 +78,8 @@
       }
     } else {
       for (i = 0; i < size; i++) {
-        arr[i] = (int32_t)((int64_t)arr[i] * (1ULL << (-bit)));
+        arr[i] = (int32_t)clamp64((int64_t)arr[i] * (1 << (-bit)), INT32_MIN,
+                                  INT32_MAX);
       }
     }
   }