Validate correct string data in fuzz test

include/weaseljson.h

@@ -1,7 +1,7 @@
 #ifndef WEASELJSON_H
 #define WEASELJSON_H
 
-struct Callbacks {
+struct WeaselJsonCallbacks {
   void (*on_begin_object)(void *data);
   void (*on_end_object)(void *data);
   void (*on_begin_string)(void *data);