chore: move put cache after signature verification (#2223)

Author: rishav-karanjit

DynamoDbEncryption/runtimes/java/src/main/sdkv2/com/amazonaws/services/dynamodbv2/datamodeling/sdkv2/encryption/DynamoDBEncryptor.java

@@ -355,6 +355,7 @@ public Map<String, AttributeValue> decryptRecord(
       materials.getVerificationKey(),
       signature
     );
+    DynamoDbSigner.putCacheIfEmpty(signer);
     itemAttributes.remove(materialDescriptionFieldName);
 
     actualDecryption(

DynamoDbEncryption/runtimes/java/src/main/sdkv2/com/amazonaws/services/dynamodbv2/datamodeling/sdkv2/encryption/DynamoDbSigner.java

@@ -62,11 +62,14 @@ static DynamoDbSigner getInstance(String signingAlgorithm, SecureRandom rnd) {
     DynamoDbSigner result = cache.get(signingAlgorithm);
     if (result == null) {
       result = new DynamoDbSigner(signingAlgorithm, rnd);
-      cache.putIfAbsent(signingAlgorithm, result);
     }
     return result;
   }
 
+  static void putCacheIfEmpty(DynamoDbSigner signer) {
+    cache.putIfAbsent(signer.getSigningAlgorithm(), signer);
+  }
+
   /**
    * @param signingAlgorithm is the algorithm used for asymmetric signing (ex: SHA256withRSA). This
    *     is ignored for symmetric HMACs as that algorithm is fully specified by the key.