mbedtls: Avoid one more memory leak

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Eshan Kelkar <eshankelkar@galorithm.com>
(cherry picked from commit 4f239f79c6)

src/pki_mbedcrypto.c

@@ -1075,7 +1075,7 @@ ssh_string pki_key_to_blob(const ssh_key key, enum ssh_key_e type)
 
             if (d == NULL) {
                 SSH_BUFFER_FREE(buffer);
-                return NULL;
+                goto out;
             }
 
             rc = ssh_buffer_add_ssh_string(buffer, d);