Use direct assignment for string in class initialization

Change string initialization to use direct assignment instead of
a class method in the GParted_Core class initialization.

Thanks go to Jan Claeys for pointing out this potential problem
and providing the following link:
https://bugzilla.gnome.org/show_bug.cgi?id=627724#c7

This link contains a comment by Matthias Clasen that says:

    "In a class init function, don't do anything that causes
    other threads to initialize classed types."

src/GParted_Core.cc

@@ -63,7 +63,7 @@ GParted_Core::GParted_Core()
 	lp_disk = NULL ;
 	lp_partition = NULL ;
 	p_filesystem = NULL ;
-	set_thread_status_message("") ;
+	thread_status_message = "" ;
 
 	ped_exception_set_handler( ped_exception_handler ) ;