examples/echo: free allocated memory when an error happens

If either `priv_key` or `pub_key` is `NULL` but not the other one, the
allocated memory is not freed. This issue is reported by clang's static
analyzer.

Fix this by calling `noise_free` in the error block.

examples/echo/echo-keygen/echo-keygen.c

@@ -80,6 +80,9 @@ int main(int argc, char *argv[])
     pub_key = (uint8_t *)malloc(pub_key_len);
     if (!priv_key || !pub_key) {
         fprintf(stderr, "Out of memory\n");
+        noise_free(priv_key, priv_key_len);
+        noise_free(pub_key, pub_key_len);
+        noise_dhstate_free(dh);
         return 1;
     }
     err = noise_dhstate_get_keypair