sysupgrade: fixes broken pipe error during sysupgade

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

SVN-Revision: 36548

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

@@ -157,14 +157,14 @@ get_image() { # <source> [ <command> ]
 		*) cmd="cat";;
 	esac
 	if [ -z "$conc" ]; then
-		local magic="$(eval $cmd $from | dd bs=2 count=1 2>/dev/null | hexdump -n 2 -e '1/1 "%02x"')"
+		local magic="$(eval $cmd $from 2>/dev/null | dd bs=2 count=1 2>/dev/null | hexdump -n 2 -e '1/1 "%02x"')"
 		case "$magic" in
 			1f8b) conc="zcat";;
 			425a) conc="bzcat";;
 		esac
 	fi
 
-	eval "$cmd $from ${conc:+| $conc}"
+	eval "$cmd $from 2>/dev/null ${conc:+| $conc}"
 }
 
 get_magic_word() {