Some bootloaders are broken enough to expose an ATAG_MEM with
a null size. Converting such tag to a memory node leads to
an unbootable system.

Skip over zero sized ATAG_MEM to avoid this situation.
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>