Avoid NPE during JNDI lookup

Fixed an issue where an extra line of the stack trace was being printed by the ThrowableProxyConverter when specifying a length option.

The DataSourceConnectionSource.start() method opens a new connection
only to determine whether or not the discoverConnectionProperties()
should be called. This is unnecessary since that method will perform
the same check and issue a very similar warning. A more serious
problem is that the connection is never closed.

This commit removes the use of this extra connection completely. I
think this is the last issue in need of a fix before LOGBACK-376 can
be closed.

Wrapped the call to System.getProperty in a try/catch to anticipate
the SecurityException that can result from accessing system properties
in the presence of a SecurityManager (e.g. in an applet sandbox).

Avoid NPE when getting CodeSource

CodeSource can be null - add a check to avoid NPE