fdt: fix 'prop (...) not found!' error in 'fdt set' command

This commit brings things back to the well known working state of the
command.
-
With commit 9620d87259572ef21f0df60988d9a932ca673779
(cmd/fdt: support single value replacement within an array)

there was an error introduced modifying (inserting) a property to a
device-tree node.
fdt_getprop(...) returnes a len with -1 for a non-existing property, but
a memcpy with len -1 isn't a good idea and things went wrong (crash).
-
Some times later Tom did repair this
with commit 99bb38e2cce9d99238458e0f6d1880c6d2e80a4d
(fdt: Check for NULL return from fdt_getprop in 'fdt set')

This repairs the crash but the behaviour of the command isn't like
before, it makes it impossible to insert a property.
-

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Simon Glass <sjg@chromium.org>

cmd/fdt.c

@@ -284,16 +284,14 @@ static int do_fdt(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 			len = 0;
 		} else {
 			ptmp = fdt_getprop(working_fdt, nodeoffset, prop, &len);
-			if (!ptmp) {
-				printf("prop (%s) not found!\n", prop);
-				return 1;
-			}
 			if (len > SCRATCHPAD) {
 				printf("prop (%d) doesn't fit in scratchpad!\n",
 				       len);
 				return 1;
 			}
-			memcpy(data, ptmp, len);
+			if (ptmp != NULL)
+				memcpy(data, ptmp, len);
+
 			ret = fdt_parse_prop(&argv[4], argc - 4, data, &len);
 			if (ret != 0)
 				return ret;