Fix bug with absolute meshdirs in included XML files. Fixes #1772.

PiperOrigin-RevId: 649150979
Change-Id: I5df08c689afc4f5bc625e989a8c0b9733fcfa3c2

src/xml/xml_util.cc

@@ -168,7 +168,7 @@ static std::string ResolveFilePath(XMLElement* e, std::string filename,
 
   // TODO(kylebayes): We first look in the base model directory for files to
   // remain backwards compatible.
-  std::string full_filename = dir + filename;
+  std::string full_filename = mjuu_combinePaths(dir, filename);
   mjResource *resource = mju_openResource(full_filename.c_str(), nullptr, 0);
   if (resource != nullptr) {
     mju_closeResource(resource);
@@ -185,7 +185,7 @@ static std::string ResolveFilePath(XMLElement* e, std::string filename,
       break;
     }
   }
-  return path + filename;
+  return mjuu_combinePaths(path, filename);
 }
 
 // constructor