Eip712 review (#355)

* Possible security fix.

It is possible to send a new structure definition after sending a structure implementation, which makes the app treat unrestricted data as if it was a well defined structure.
This commit tries to fix that behaviour. Once a structure implementation is sent, we consider all structures to be defined and we do not allow new definitions.

* Fix previous commit

src_features/signMessageEIP712/commands_712.c

@@ -52,6 +52,11 @@ bool handle_eip712_struct_def(const uint8_t *const apdu_buf) {
     if (eip712_context == NULL) {
         ret = eip712_context_init();
     }
+
+    if (struct_state == DEFINED) {
+        ret = false;
+    }
+
     if (ret) {
         switch (apdu_buf[OFFSET_P2]) {
             case P2_DEF_NAME: