SeedPeers: fix conversion of int representation of IPv4 addresses to InetAddress

Usage of `writeUint32LE()` is wrong on two counts:

* `InetAddress.getByAddress()` expects big-endian values, and the method
  above encodes as little-endian.
* Java int is signed, even if the Java hex literal appears unsigned. The
  above method expects unsigned values.

So we add the previously missing `writeInt32BE()` to ByteUtils and use it.

core/src/main/java/org/bitcoinj/base/internal/ByteUtils.java

@@ -203,6 +203,31 @@ public class ByteUtils {
         writeUint32BE(val, ByteBuffer.wrap(out, offset, out.length - offset));
     }
 
+    /**
+     * Write 4 bytes to the buffer as signed 32-bit integer in big endian format.
+     * @param val value to be written
+     * @param buf buffer to be written into
+     * @return the buffer
+     * @throws BufferOverflowException if the value doesn't fit the remaining buffer
+     */
+    public static ByteBuffer writeInt32BE(int val, ByteBuffer buf) throws BufferOverflowException {
+        return buf.order(ByteOrder.BIG_ENDIAN).putInt((int) val);
+    }
+
+    /**
+     * Write 4 bytes to the byte array (starting at the offset) as signed 32-bit integer in big endian format.
+     * @param val value to be written
+     * @param out buffer to be written into
+     * @param offset offset into the buffer
+     * @throws ArrayIndexOutOfBoundsException if offset points outside of the buffer, or
+     *                                        if the value doesn't fit the remaining buffer
+     */
+    public static void writeInt32BE(int val, byte[] out, int offset) throws ArrayIndexOutOfBoundsException {
+        check(offset >= 0 && offset <= out.length - 4, () ->
+                new ArrayIndexOutOfBoundsException(offset));
+        writeInt32BE(val, ByteBuffer.wrap(out, offset, out.length - offset));
+    }
+
     /**
      * Write 8 bytes to the buffer as signed 64-bit integer in little endian format.
      * @param val value to be written

core/src/main/java/org/bitcoinj/net/discovery/SeedPeers.java

@@ -122,7 +122,7 @@ public class SeedPeers implements PeerDiscovery {
 
     private static InetAddress convertAddress(int seed) throws UnknownHostException {
         byte[] v4addr = new byte[4];
-        ByteUtils.writeUint32LE(seed, v4addr, 0);
+        ByteUtils.writeInt32BE(seed, v4addr, 0);
         return InetAddress.getByAddress(v4addr);
     }
 }