[MINOR] Fix bounds check in PacketType. (#132)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

ethereum/p2p/src/main/java/tech/pegasys/pantheon/ethereum/p2p/discovery/internal/PacketType.java

@@ -39,7 +39,7 @@ public enum PacketType {
   private final Deserializer<?> deserializer;
 
   public static Optional<PacketType> forByte(final byte b) {
-    return b > MAX_VALUE ? Optional.empty() : Optional.ofNullable(INDEX[b]);
+    return b >= MAX_VALUE || b < 0 ? Optional.empty() : Optional.ofNullable(INDEX[b]);
   }
 
   PacketType(final int value, final Deserializer<?> deserializer) {

ethereum/p2p/src/test/java/tech/pegasys/pantheon/ethereum/p2p/discovery/internal/PacketTypeTest.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2018 ConsenSys AG.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+package tech.pegasys.pantheon.ethereum.p2p.discovery.internal;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class PacketTypeTest {
+
+  @Test
+  public void shouldReturnEmptyPacketTypeForNegativeByte() {
+    assertThat(PacketType.forByte((byte) -1)).isEmpty();
+  }
+
+  @Test
+  public void shouldReturnEmptyPacketTypeForByteAboveMaxValue() {
+    assertThat(PacketType.forByte(Byte.MAX_VALUE)).isEmpty();
+  }
+
+  @Test
+  public void shouldWorkForEveryPossibleByteValue() {
+    for (int b = Byte.MIN_VALUE; b <= Byte.MAX_VALUE; b++) {
+      assertThat(PacketType.forByte((byte) b)).isNotNull();
+    }
+  }
+
+  @Test
+  public void shouldReturnEachPacketTypeByByte() {
+    for (final PacketType packetType : PacketType.values()) {
+      assertThat(PacketType.forByte(packetType.getValue())).contains(packetType);
+    }
+  }
+}