Zeroize static ephemeral key buffer before free

F-2144

SetStaticEphemeralKey loaded a private key file into keyBuf and freed it
without ForceZero. Static ephemeral keys are long-lived, so zeroize the
buffer before XFREE.

src/ssl.c

@@ -18645,6 +18645,7 @@ static int SetStaticEphemeralKey(WOLFSSL_CTX* ctx,
 #ifndef NO_FILESYSTEM
     /* done with keyFile buffer */
     if (keyFile && keyBuf) {
+        ForceZero(keyBuf, keySz);
         XFREE(keyBuf, heap, DYNAMIC_TYPE_TMP_BUFFER);
     }
 #endif