fdt: Add a check to do_fdt() for coverity

We know that fdt_getprop() does not return NULL when len is > 0 but
coverity does not. Add an extra check to keep it happy.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 163249)
Fixes: bc80295b (fdt: Add get commands to fdt)