Java synchronized blocks don't throw NullPointerException

[reuk]

The docs for the Java monitorenter instruction say that NullPointerException should be thrown if the argument to the synchronized block is null.

It seems like the current behaviour is to ignore monitorenter and monitorexit (according to #186), which leads to inaccurate results. I suggest that the exception-throwing behaviour is modelled, even if the thread-synchronization itself is not.

[peterschrammel]

The monitor entry exception is fixed in #3423.

The Monitor exit exceptions require additional attention: Enabling these exceptions makes jbmc-regression/synchronized-blocks/test_sync.desc run into an infinite loop during symex (which I don't have time to investigate now).