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

In 1fec3c5 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>

common/image.c

@@ -913,7 +913,7 @@ int boot_get_ramdisk(int argc, char * const argv[], bootm_headers_t *images,
 	 * Look for an Android boot image.
 	 */
 	buf = map_sysmem(images->os.start, 0);
-	if (genimg_get_format(buf) == IMAGE_FORMAT_ANDROID)
+	if (buf && genimg_get_format(buf) == IMAGE_FORMAT_ANDROID)
 		select = argv[0];
 #endif