SITL: ensure st.size is valid when loading model json

Before this patch st.size is undefined when we use it to create a buffer
on the stack - probably not a good thing.

libraries/SITL/SIM_Frame.cpp

@@ -333,6 +333,9 @@ void Frame::load_frame_params(const char *model_json)
         fname = strdup(model_json);
     } else {
         IGNORE_RETURN(asprintf(&fname, "@ROMFS/models/%s", model_json));
+        if (AP::FS().stat(model_json, &st) != 0) {
+            AP_HAL::panic("%s failed to load\n", model_json);
+        }
     }
     if (fname == nullptr) {
         AP_HAL::panic("%s failed to load\n", model_json);