ARM: zImage: don't ignore error returned from decompress()

If decompress() returns an error without calling error(), we must
not attempt to boot the resulting kernel.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Tony Lindgren <tony@atomide.com>