src: add default value for RSACipherConfig mode field

Using default init of enum is UB

Refs: #56693
PR-URL: #56701
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/crypto/crypto_rsa.h

@@ -77,7 +77,7 @@ struct RSAKeyExportTraits final {
 using RSAKeyExportJob = KeyExportJob<RSAKeyExportTraits>;
 
 struct RSACipherConfig final : public MemoryRetainer {
-  CryptoJobMode mode;
+  CryptoJobMode mode = kCryptoJobAsync;
   ByteSource label;
   int padding = 0;
   const EVP_MD* digest = nullptr;