src: assert return value of BN_bn2binpad

Every other invocation of BN_bn2binpad checks the return value. For
safety and consistency, do so in RandomPrimeTraits::EncodeOutput()
as well.

src/crypto/crypto_random.cc

@@ -75,10 +75,10 @@ Maybe<bool> RandomPrimeTraits::EncodeOutput(
   size_t size = BN_num_bytes(params.prime.get());
   std::shared_ptr<BackingStore> store =
       ArrayBuffer::NewBackingStore(env->isolate(), size);
-  BN_bn2binpad(
-      params.prime.get(),
-      reinterpret_cast<unsigned char*>(store->Data()),
-      size);
+  CHECK_EQ(static_cast<int>(size),
+           BN_bn2binpad(params.prime.get(),
+                        reinterpret_cast<unsigned char*>(store->Data()),
+                        size));
   *result = ArrayBuffer::New(env->isolate(), store);
   return Just(true);
 }