Fixed several findings from LGTM.com

- Fixed a few possible null dereferences
- Fixed a few possible out-of-bound array ops
- Added a couple of test cases

httpclient5/src/main/java/org/apache/hc/client5/http/entity/mime/HttpRFC7578Multipart.java

@@ -137,13 +137,12 @@ class HttpRFC7578Multipart extends AbstractMultipartFormat {
             for (int i = 0; i < bytes.length; i++) {
                 final int b = bytes[i];
                 if (b == ESCAPE_CHAR) {
-                    try {
-                        final int u = digit16(bytes[++i]);
-                        final int l = digit16(bytes[++i]);
-                        buffer.append((char) ((u << 4) + l));
-                    } catch (final ArrayIndexOutOfBoundsException e) {
-                        throw new DecoderException("Invalid URL encoding: ", e);
+                    if (i >= bytes.length - 2) {
+                        throw new DecoderException("Invalid URL encoding: too short");
                     }
+                    final int u = digit16(bytes[++i]);
+                    final int l = digit16(bytes[++i]);
+                    buffer.append((char) ((u << 4) + l));
                 } else {
                     buffer.append(b);
                 }

httpclient5/src/main/java/org/apache/hc/client5/http/impl/io/BasicHttpClientConnectionManager.java

@@ -275,7 +275,9 @@ public class BasicHttpClientConnectionManager implements HttpClientConnectionMan
                 }
             } else {
                 this.state = state;
-                conn.passivate();
+                if (conn != null) {
+                    conn.passivate();
+                }
                 if (TimeValue.isPositive(keepAlive)) {
                     if (LOG.isDebugEnabled()) {
                         LOG.debug("Connection can be kept alive for {}", keepAlive);

httpclient5/src/test/java/org/apache/hc/client5/http/entity/mime/HttpRFC7578MultipartTest.java

@@ -0,0 +1,60 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+
+package org.apache.hc.client5.http.entity.mime;
+
+import org.apache.commons.codec.DecoderException;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class HttpRFC7578MultipartTest {
+
+    @Test(expected = DecoderException.class)
+    public void testPercentDecodingWithTooShortMessage() throws DecoderException {
+        new HttpRFC7578Multipart.PercentCodec().decode("%".getBytes());
+    }
+
+    @Test
+    public void testPercentDecodingWithValidMessages() throws DecoderException {
+        final HttpRFC7578Multipart.PercentCodec codec = new HttpRFC7578Multipart.PercentCodec();
+        final String[][] tests = new String[][] {
+                {"test", "test"},
+                {"%20", " "},
+                {"a%20b", "a b"},
+                {"https%3A%2F%2Fhc.apache.org%2Fhttpcomponents-client-5.0.x%2Findex.html",
+                        "https://hc.apache.org/httpcomponents-client-5.0.x/index.html"},
+                {"%00", "\00"},
+                {"%0A", "\n"},
+
+        };
+        for (final String[] test : tests) {
+            assertEquals(test[1], new String(codec.decode(test[0].getBytes())));
+        }
+    }
+
+}