Zeroize RSA DER buffer in CTX_use_RSAPrivateKey before free

F-2145

wolfSSL_CTX_use_RSAPrivateKey staged the RSA private key DER (PKCS#1:
n, e, d, p, q, dP, dQ, qInv) in a heap buffer and freed it without
ForceZero. Zeroize before XFREE.

Author: julek-wolfssl

src/ssl_load.c

@@ -5365,6 +5365,9 @@ int wolfSSL_CTX_use_RSAPrivateKey(WOLFSSL_CTX* ctx, WOLFSSL_RSA* rsa)
     }
 
     /* Dispos of dynamically allocated data. */
+    if (der != NULL) {
+        ForceZero(der, (word32)derSize);
+    }
     XFREE(der, NULL, DYNAMIC_TYPE_TMP_BUFFER);
     return ret;
 }