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/path.c

@@ -363,6 +363,8 @@ bool path_set_root(const char *const struct_name, uint8_t name_length) {
         path_struct->root_type = ROOT_MESSAGE;
     }
 
+    struct_state = DEFINED;
+
     // because the first field could be a struct type
     path_update();
     return true;