Check for integer overflow in AesTask::DoCrypto() (DiD)

After calling mResult.SetLength(mData.Length() + 16) we should check
that the integer addition didn't overflow. It seems at the moment
impossible to create ArrayBuffers of size >= 0x0xfffffff0, however
adding a check here doesn't hurt.

mResult.Length() is passed to the PK11 API functions as a
maxOut parameter and should be checked by the
softoken crypto algorithm implementations.
AES-ECB and AES-GCM seem to do that correctly.

dom/crypto/WebCryptoTask.cpp

@@ -716,6 +716,11 @@ private:
       return NS_ERROR_DOM_INVALID_ACCESS_ERR;
     }
 
+    // Check whether the integer addition would overflow.
+    if (std::numeric_limits<CryptoBuffer::size_type>::max() - 16 < mData.Length()) {
+      return NS_ERROR_DOM_DATA_ERR;
+    }
+
     // Initialize the output buffer (enough space for padding / a full tag)
     uint32_t dataLen = mData.Length();
     uint32_t maxLen = dataLen + 16;