screen-share: Use the correct list pointer when freeing shm buffers
screen-share: Use the correct list pointer when freeing shm buffers

The wl_list_for_each operation on the free_buffers list should use
free_link not link, which is a different list.

Signed-off-by: Bryce Harrington <bdb14cfc1ab8d24e13de84adc40b41ae60c13bd1@osg.samsung.com>
Reviewed-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>

giucam/weston,Fantu/compositor-spice,sir-murray/weston,sir-murray/weston,jonnylamb/weston,giucam/weston,eyolfson/weston,Gnurou/weston,krezovic/weston,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,xorgy/weston,Gnurou/weston,kwm81/weston,jonnylamb/weston,kwm81/weston,krezovic/weston,Fantu/compositor-spice,eyolfson/weston,udoprog/weston,udoprog/weston,xorgy/weston