Fix a NullPointerException if properties file doesn't exist

For example with following URL,

  amazon-s3://.jgit@mybucket/foo.git

if ~/.jgit is missing, jgit command will throw a NullPointerException.

With this patch, a reasonable error message will be emitted:

  fatal: Cannot read file /Users/jamesyonan/.jgit

Change-Id: I1d366f2d55e170d2972715c657c8e2d10c8d87d2
Signed-off-by: James Yonan <james@openvpn.net>

org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportAmazonS3.java

@@ -170,8 +170,14 @@ public class TransportAmazonS3 extends HttpTransport implements WalkTransport {
 			return loadPropertiesFile(propsFile);
 
 		Properties props = new Properties();
-		props.setProperty("accesskey", uri.getUser()); //$NON-NLS-1$
-		props.setProperty("secretkey", uri.getPass()); //$NON-NLS-1$
+		String user = uri.getUser();
+		String pass = uri.getPass();
+		if (user != null && pass != null) {
+		        props.setProperty("accesskey", user); //$NON-NLS-1$
+		        props.setProperty("secretkey", pass); //$NON-NLS-1$
+		} else
+			throw new NotSupportedException(MessageFormat.format(
+					JGitText.get().cannotReadFile, propsFile));
 		return props;
 	}