Fix case where an exception (indirectly) supresses itself

Author: SalusaSecondus

src/main/java/com/amazonaws/encryptionsdk/MasterKeyProvider.java

@@ -102,10 +102,13 @@ protected AwsCryptoException buildCannotDecryptDksException(List<? extends Throw
         if (t == null || t.isEmpty()) {
             return new CannotUnwrapDataKeyException("Unable to decrypt any data keys");
         } else {
-            final CannotUnwrapDataKeyException ex = new CannotUnwrapDataKeyException("Unable to decrypt any data keys",
-                    t.get(0));
+            CannotUnwrapDataKeyException ex = null;
             for (final Throwable e : t) {
-                ex.addSuppressed(e);
+                if (ex == null) {
+                    ex = new CannotUnwrapDataKeyException("Unable to decrypt any data keys", e);
+                } else {
+                    ex.addSuppressed(e);
+                }
             }
             return ex;
         }