Ensure decoder has an error string.

We use that error buffer to check for errors.

PiperOrigin-RevId: 889193796
Change-Id: I1f7575321a57301b4ae5002d0c488bbb16a30b26

src/experimental/mjz/mjz_decoder.cc

@@ -206,8 +206,9 @@ mjPLUGIN_LIB_INIT {
     if (size <= 0) {
       return nullptr;
     }
+    char error[1024];
     return ParseZipBuffer(buffer, size, resource->name, const_cast<mjVFS*>(vfs),
-                          nullptr, 0);
+                          error, sizeof(error));
   };
   mjp_registerDecoder(&decoder);
 }