fdt: Check for NULL return from fdt_getprop in 'fdt set'

While the previous pass through fixed one place where we knew that
fdt_getprop would be given a positive len, in the case of 'fdt set' we
do not, so check that we did no get NULL from fdt_getprop().

Cc: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 163249)
Fixes 72c98ed1

 ("fdt: Add a check to do_fdt() for coverity")
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>