base-files: sysupgrade: abort if config backup fails

Sysupgrade shouldn't proceed, if the backup of the configuration
fails because tar (or gzip) exit with a non-zero code.

Signed-off-by: Andreas Ziegler <dev@andreas-ziegler.de>

package/base-files/files/sbin/sysupgrade

@@ -165,6 +165,11 @@ do_save_conffiles() {
 	v "Saving config files..."
 	[ "$VERBOSE" -gt 1 ] && TAR_V="v" || TAR_V=""
 	tar c${TAR_V}zf "$conf_tar" -T "$CONFFILES" 2>/dev/null
+	if [ "$?" -ne 0 ]; then
+		echo "Failed to create the configuration backup."
+		rm -f "$conf_tar"
+		exit 1
+	fi
 
 	rm -f "$CONFFILES"
 }