libfdt: Propagate name errors in fdt_getprop_by_offset()

fdt_getprop_by_offset() doesn't check for errors from fdt_string() - after
all, until very recently it couldn't fail.  Now it can, so we need to
propagate errors up to the caller.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Simon Glass <sjg@chromium.org>