gif: do not set avctx->coded_frame

Signed-off-by: Paul B Mahol <onemda@gmail.com>
gif: remove unused AVFrame from encoder private context

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Check the return value of ff_rv34_decode_init() in rv30.c and rv40.c

Avoids possible null pointer dereferences on oom.

Fixes ticket #2727.