Fix 2 heap-buffer-overflows in trw_layer_download_map_along_track_cb()

1 byte is less than sizeof(gpointer), so we do not have enough memory
for last NULL pointer.

This could be solved by braces, but since glib provides g_malloc_n(),
and this kind of bug can be avoided with it, so.

Signed-off-by: Rob Norris <rw_norris@hotmail.com>