image.c: Fix non-Android booting with ramdisk and/or device tree

In 1fec3c5d

 I added a check that if we had an Android image we default to
trying the kernel address for a ramdisk.  However when we don't have an
Android image buf is NULL and we oops here.  Ensure that we have 'buf'
to check first.

Reported-by: elipe Balbi <balbi@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>