Validate correct string data in fuzz test

src/parser3.h

@@ -74,7 +74,7 @@ enum Symbol : uint8_t {
   N_SYMBOL_COUNT, // Must be last
 };
 struct Parser3 {
-  Parser3(const Callbacks *callbacks, void *data)
+  Parser3(const WeaselJsonCallbacks *callbacks, void *data)
       : callbacks(callbacks), data(data) {
     std::ignore = push({N_WHITESPACE, N_VALUE, N_WHITESPACE, T_EOF});
   }
@@ -138,7 +138,7 @@ struct Parser3 {
   char *dataBegin;
   // Used for unescaping string data in place
   char *writeBuf;
-  const Callbacks *const callbacks;
+  const WeaselJsonCallbacks *const callbacks;
   void *const data;
   Symbol stack[kMaxStackSize];
   Symbol *stackPtr = stack;