Fix building for 32bit architectures

The code was previously assuming that int type was always int64. This
caused us to fail to build for 32bit systems where we were doing a
check against MaxUint32.

We now explicitly cast the value to int64 prior to doing the check. On
32bit systems this is fine because the length of the array can't be
larger than MaxInt32.

Fixes: #1

wormhole/file_transport.go

@@ -137,7 +137,11 @@ func (d *transportCryptor) writeRecord(msg []byte) error {
 
 	nonceAndSealedMsg := append(nonce[:], sealedMsg...)
 
-	if len(nonceAndSealedMsg) >= math.MaxUint32 {
+	// we do an explit cast to int64 to avoid compilation failures
+	// for 32bit systems.
+	nonceAndSealedMsgSize := int64(len(nonceAndSealedMsg))
+
+	if nonceAndSealedMsgSize >= math.MaxUint32 {
 		panic(fmt.Sprintf("writeRecord too large: %d", len(nonceAndSealedMsg)))
 	}