screenshooter: fix various memory handling

There were unchecked malloc and no free for this memory.
Also simplify error handling in one function.

v2. remove check if memory is NULL, according to man pages,
    free(NULL) is a no-op

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
1 file changed