Throw IOE instead of NPE if OpenSSHKeyV1KeyFile reads an empty file

There is a contract that FileKeyProvider.readKey throws an IOException if something goes wrong. Throwing an NPE is not expected by API users. Also, it is much more difficult to find out if the NPE is thrown due to a broken key file, or due to an internal bug.

src/main/java/com/hierynomus/sshj/userauth/keyprovider/OpenSSHKeyV1KeyFile.java

@@ -218,6 +218,9 @@ private boolean checkHeader(final BufferedReader reader) throws IOException {
         while (line != null && !line.startsWith(BEGIN)) {
             line = reader.readLine();
         }
+        if (line == null) {
+            return false;
+        }
         line = line.substring(BEGIN.length());
         return line.startsWith(OPENSSH_PRIVATE_KEY);
     }

src/test/java/net/schmizz/sshj/keyprovider/OpenSSHKeyFileTest.java

@@ -39,6 +39,7 @@
 import java.io.IOException;
 import java.io.InputStreamReader;
 import java.io.OutputStreamWriter;
+import java.io.StringReader;
 import java.math.BigInteger;
 import java.security.GeneralSecurityException;
 import java.security.PrivateKey;
@@ -443,6 +444,14 @@ public void notTrimmedKeys() throws IOException {
                      corruptedKeyFile.getPublic());
     }
 
+    @Test
+    public void emptyPrivateKey() {
+        FileKeyProvider keyProvider = new OpenSSHKeyV1KeyFile();
+        keyProvider.init(new StringReader(""));
+
+        assertThrows("This key is not in 'openssh-key-v1' format", IOException.class, keyProvider::getPrivate);
+    }
+
     @Before
     public void checkBCRegistration() {
         if (!SecurityUtils.isBouncyCastleRegistered()) {