build-scripts/clean_obs.sh: Fix a bug.

obs/generic/build-scripts/clean_obs.sh

@@ -5,7 +5,9 @@ source build-scripts/configuration_information.sh
 
 # reverse the action of obsoleting the potentially altered buildout and restoring the backed up one
 if [ ! -e $RUN_BUILDOUT_DIR/bin/backup.buildout ]; then
-	cp $RUN_BUILDOUT_DIR/bin/buildout $RUN_BUILDOUT_DIR/bin/backup.buildout
+	if [ -e $RUN_BUILDOUT_DIR/bin/buildout ]; then
+		cp $RUN_BUILDOUT_DIR/bin/buildout $RUN_BUILDOUT_DIR/bin/backup.buildout
+	fi
 fi
 if [ -e $RUN_BUILDOUT_DIR/bin/old.buildout ]; then
 	mv $RUN_BUILDOUT_DIR/bin/old.buildout $RUN_BUILDOUT_DIR/bin/buildout