fate: update mmf seek checksum, change caused by av_get_packet() useage

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

tests/ref/seek/lavf_mmf

-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:     -1 size:  4096
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:     81 size:  4096
 ret:-1         st:-1 flags:0  ts:-1.000000
 ret:-1         st:-1 flags:1  ts: 1.894167
 ret:-1         st: 0 flags:0  ts: 0.788345