base-files: properly remove stderr from zcat/bzcat

Signed-off-by: John Crispin <blogic@openwrt.org>

SVN-Revision: 41135

package/base-files/files/lib/upgrade/common.sh

@@ -182,11 +182,11 @@ get_image() { # <source> [ <command> ]
 }
 
 get_magic_word() {
-	get_image "$@" | dd bs=2 count=1 2>/dev/null | hexdump -v -n 2 -e '1/1 "%02x"'
+	(get_image "$@" | dd bs=2 count=1 | hexdump -v -n 2 -e '1/1 "%02x"') 2>/dev/null
 }
 
 get_magic_long() {
-	get_image "$@" | dd bs=4 count=1 2>/dev/null | hexdump -v -n 4 -e '1/1 "%02x"'
+	(get_image "$@" | dd bs=4 count=1 | hexdump -v -n 4 -e '1/1 "%02x"') 2>/dev/null
 }
 
 jffs2_copy_config() {