draw: avoid leaking tokens when building pstipple fragment shader

Add missing FREE() after MALLOC().

src/gallium/auxiliary/draw/draw_pipe_pstipple.c

@@ -358,6 +358,7 @@ generate_pstip_fs(struct pstip_stage *pstip)
 
    pstip->fs->pstip_fs = pstip->driver_create_fs_state(pstip->pipe, &pstip_fs);
 
+   FREE((void *)pstip_fs.tokens);
    return TRUE;
 }