flicvideo: fix crash on flic files with invalid frame size

Add a check in flic_decode_frame_8BPP(), in case chunk_size is >
frame_size issue a warning and resize chunk_size to frame_size, in
order to avoid out-of-buffer reads.

Fix roundup issue #2520, trac issue #69.

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>