The 'map' member of the level struct is an array of buffers containing
a row of map_tile structs.
The map_tile struct contains a list_head member to hold a list of
volatile obstacles.
When a list is empty, list_head.next and list_head.prev points
to the list_head.

When removing the western column of a level, all rows in level->map
are shifted left by using memmove() and realloc().
After the memory shift and (possible) memory reallocation, the
list_head.next and list_head.prev pointers no more point to the right
memory slot.

The solve the problem, this patch replaces the
map_tile.volatile_obstacles member by a pointer to a list_head, that
do not suffer any issue due to memory shifting or reallocation of
the map_tiles.

This patch also free the memory used by removed rows or columns.