src/useradd: free string

useradd.c:2329:10: warning: Potential leak of memory pointed to by 'btrfs_check' [unix.Malloc]

src/useradd.c

@@ -2319,6 +2319,7 @@ static void create_home (void)
 					Prog, path);
 				fail_exit(E_HOMEDIR);
 			}
+			free(btrfs_check);
 			// make subvolume to mount for user instead of directory
 			if (btrfs_create_subvolume(path)) {
 				fprintf(stderr,