Fixing aegis test failure in //mode/opt

Summary: Fixing [aegis test failure](https://www.internalfb.com/omh/view/secure_pipes/tests) in //mode/opt

Reviewed By: mingtaoy

Differential Revision: D47417779

fbshipit-source-id: 798de04256593010a2e8b6efc3575e5f54df12ea

Author: Huilin Chen

fizz/crypto/aead/AEGISCipher.cpp

@@ -325,14 +325,14 @@ folly::Optional<std::unique_ptr<folly::IOBuf>> AEGISCipher::tryDecrypt(
   // Set up the tag buffer now
   const auto& lastBuf = ciphertext->prev();
   folly::MutableByteRange tagOut;
+  std::array<uint8_t, kTagLength> tag;
   if (lastBuf->length() >= tagLength_) {
     // We can directly carve out this buffer from the last IOBuf
     // Adjust buffer sizes
     lastBuf->trimEnd(tagLength_);
 
     tagOut = {lastBuf->writableTail(), tagLength_};
   } else {
-    std::array<uint8_t, kTagLength> tag;
     // buffer to copy the tag into when we decrypt
     tagOut = {tag.data(), tagLength_};
     trimBytes(*ciphertext, tagOut);