graphics/php5-ffmpeg: unbreak with ffmpeg 4.0

ffmpeg_movie.c:978:29: error: no member named 'codec_name' in 'struct AVCodecContext'; did you mean 'coded_frame'?

} else if (decoder_ctx->codec_name[0] != '\0') {
                        ^~~~~~~~~~
                        coded_frame

/usr/local/include/libavcodec/avcodec.h:2760:35: note: 'coded_frame' declared here

attribute_deprecated AVFrame *coded_frame;
                              ^

ffmpeg_movie.c:978:43: error: invalid operands to binary expression ('AVFrame' (aka 'struct AVFrame') and 'int')

} else if (decoder_ctx->codec_name[0] != '\0') {
           ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~

ffmpeg_movie.c:979:35: error: no member named 'codec_name' in 'struct AVCodecContext'

codec_name = decoder_ctx->codec_name;
             ~~~~~~~~~~~  ^

PR: 227726
Reported by: antoine (via exp-run)