pthreads: Fix bug introduced with thread_safe_callbacks
For intra codecs, ff_thread_finish_setup() is called before decoding starts
automatically. However, get_buffer can only be used before it's called, so
adding this requirement broke frame threading for them. Fixed by moving the
call until after get_buffer is finished.
Signed-off-by: Ronald S. Bultje <email@example.com>