Fix to avoid undefined behavior at strcpy

- Using the `path` buffer to store the directory name, the possible overlapping areas can be avoided at strcpy.
- The `path` buffer is unused in further parts of this function, so can be used freely here. The `temp` buffer is too small for this task.

src/86box.c

@@ -564,11 +564,14 @@ usage:
 	 * At this point usr_path is perfectly initialized.
 	 * If no --vmname parameter specified we'll use the
 	 *   working directory name as the VM's name.
+	 * The directory name will stored in local buffer 
+	 *   named `path` since it's unused in further parts 
+	 *   of this function.
 	 */
     if (strlen(vm_name) == 0)
 	{
-		plat_get_dirname(vm_name, usr_path);
-		p = plat_get_filename(vm_name);
+		plat_get_dirname(path, usr_path);
+		p = plat_get_filename(path);
 		strcpy(vm_name, p);
 	}