Optimize OpenSSL backend to avoid redundant reinitialization

It is best to pass as few parameters to EVP_*Init_ex() as possible.
Passing a key, IV, or cipher will cause redundant work to happen behind
the scenes as OpenSSL doesn't check whether they have actually changed.

This avoids a malloc()/free() and redoing AES key expansion for every
message that is encrypted and decrypted.

src/PlatformCrypto.h

@@ -14,7 +14,7 @@ typedef struct _PLT_CRYPTO_CONTEXT {
     bool initialized;
     mbedtls_cipher_context_t ctx;
 #else
-    bool initialized; // Used for CBC only
+    bool initialized;
     EVP_CIPHER_CTX* ctx;
 #endif
 } PLT_CRYPTO_CONTEXT, *PPLT_CRYPTO_CONTEXT;