package/urandom-scripts: do not seed if initial seed doesn't exist

By returning a failure in the event that the initial seed doesn't exist,
we'd then skip creating a new seed, which means we'd never in fact have
an initial seed, and this script is therefore useless. Fix this by
checking for the existence of the seed file first, and just returning 0
if it's not there.

Reported-by: Nicolas Cavallari <Nicolas.Cavallari@green-communications.fr>
Reported-by: Eugen Hristev <Eugen.Hristev@microchip.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit e65e9acb20)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/urandom-scripts/S20urandom

@@ -18,6 +18,7 @@ else
 fi
 
 init_rng() {
+	[ -f "$URANDOM_SEED" ] || return 0
 	printf 'Initializing random number generator: '
 	dd if="$URANDOM_SEED" bs="$pool_size" of=/dev/urandom count=1 2> /dev/null
 	status=$?