OpenSSLRandom: throw exception when seed == null

Even though we don't really pay attention to the seed given to us, we
should throw NullPointerException since someone calling #setSeed(byte[])
with a null parameter is probably making a mistake anyway.

Bug: 20521744
Change-Id: Ib274c4abc059b6e4d3eb6cf95d30fec605284948
diff --git a/src/main/java/org/conscrypt/OpenSSLRandom.java b/src/main/java/org/conscrypt/OpenSSLRandom.java
index c25f2ee..d01eae0 100644
--- a/src/main/java/org/conscrypt/OpenSSLRandom.java
+++ b/src/main/java/org/conscrypt/OpenSSLRandom.java
@@ -26,6 +26,10 @@
 
     @Override
     protected void engineSetSeed(byte[] seed) {
+        if (seed == null) {
+            throw new NullPointerException("seed == null");
+        }
+
         // NOTE: The contract of the SecureRandomSpi does not appear to prohibit self-seeding here
         // (in addition to using the provided seed).
         selfSeedIfNotSeeded();