Fix NullReferenceException on exception cascade when CEK doesn't decrypt

Author: bartonjs

src/System.Security.Cryptography.Pkcs/src/Internal/Cryptography/Pal/AnyOS/ManagedPal.Decrypt.cs

@@ -82,7 +82,10 @@ public override unsafe ContentInfo TryDecrypt(
                     }
                     finally
                     {
-                        Array.Clear(cek, 0, cek.Length);
+                        if (cek != null)
+                        {
+                            Array.Clear(cek, 0, cek.Length);
+                        }
                     }
                 }