Fix NPE on connection close of shutdown connection

[mrotteveel]

When a connection is shutdown (e.g. using maintenanceManager.shutdownDatabase(MaintenanceManager.SHUTDOWN_FORCE, 0);), or otherwise produces an exception during some processing done during the close, FBConnection.close() may produce a NullPointerException when calling mc.close(this);, as mc was set to null internally by the exception handler.

This is already fixed in Jaybird 6 by some refactoring done there. Applying the same changes in Jaybird 5 should prevent this issue.