avcodec/h264_slice: Do not attempt to render into frames already output

Fixes: null pointer dereference
Fixes: 4698/clusterfuzz-testcase-minimized-5096956322906112

This testcase does not reproduce the issue before 03b82b3ab9883cef017e513c7d0b3b986b3b3e7b

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
