Add utility function for merging directory names with filenames, and fix issue in mj_addfileVFS where directory names without trailing filesystem separators won't append correctly.

PiperOrigin-RevId: 545450158
Change-Id: I550394f11ac2837d35f3d869f8718a119a9eac38

src/engine/engine_vfs.c

@@ -15,7 +15,6 @@
 #include "engine/engine_vfs.h"
 
 #include <string.h>
-#include <stdlib.h>
 
 #include "engine/engine_array_safety.h"
 #include "engine/engine_plugin.h"
@@ -72,11 +71,8 @@ int mj_addFileVFS(mjVFS* vfs, const char* directory, const char* filename) {
 
   // make full name
   char fullname[1000];
-  if (directory) {
-    mjSTRNCPY(fullname, directory);
-    mjSTRNCAT(fullname, filename);
-  } else {
-    mjSTRNCPY(fullname, filename);
+  if (mju_makefullname(fullname, sizeof(fullname), directory, filename)) {
+    return -1;
   }
 
   // strip path