Bugfix: copying drivers did not set compiled expression to NULL, also tweak

to set it to NULL on file read instead of write as is done usually.

source/blender/blenkernel/intern/fcurve.c

@@ -1205,6 +1205,7 @@ ChannelDriver *fcurve_copy_driver (ChannelDriver *driver)
 		
 	/* copy all data */
 	ndriver= MEM_dupallocN(driver);
+	ndriver->expr_comp= NULL;
 	
 	/* copy variables */
 	ndriver->variables.first= ndriver->variables.last= NULL;

source/blender/blenloader/intern/readfile.c

@@ -1780,6 +1780,8 @@ static void direct_link_fcurves(FileData *fd, ListBase *list)
 		if (fcu->driver) {
 			ChannelDriver *driver= fcu->driver;
 			DriverVar *dvar;
+
+			driver->expr_comp= NULL;
 			
 			/* relink variables, targets and their paths */
 			link_list(fd, &driver->variables);

source/blender/blenloader/intern/writefile.c

@@ -935,15 +935,8 @@ static void write_fcurves(WriteData *wd, ListBase *fcurves)
 			ChannelDriver *driver= fcu->driver;
 			DriverVar *dvar;
 			
-			/* don't save compiled python bytecode */
-			void *expr_comp= driver->expr_comp;
-			driver->expr_comp= NULL;
-			
 			writestruct(wd, DATA, "ChannelDriver", 1, driver);
 			
-			driver->expr_comp= expr_comp; /* restore */
-			
-			
 			/* variables */
 			for (dvar= driver->variables.first; dvar; dvar= dvar->next) {
 				writestruct(wd, DATA, "DriverVar", 1, dvar);