Avoid out-of-bounds access when checking file extensions.

Include the '.' in the file extension check.

PiperOrigin-RevId: 474866478
Change-Id: I809ae975e4c8b00f17278735975a7d07e0748643

src/user/user_util.h

@@ -16,6 +16,7 @@
 #define MUJOCO_SRC_USER_USER_UTIL_H_
 
 #include <string>
+#include <string_view>
 
 
 extern const double mjNAN;                      // used to mark undefined fields
@@ -129,6 +130,9 @@ std::string mjuu_strippath(std::string filename);
 // strip extension from filename
 std::string mjuu_stripext(std::string filename);
 
+// get the extension of a filename
+std::string mjuu_getext(std::string_view filename);
+
 // check if path is absolute
 bool mjuu_isabspath(std::string path);