lavf/dump: dump the vbv_delay with N/A instead of 18446744073709551615

lavf/dump: dump the vbv_delay with N/A instead of 18446744073709551615

How to check it:
./ffmpeg -f lavfi -i testsrc  -c:v mpeg2video  -f null  -
master:
  Side data:
        cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay:
        18446744073709551615
patch applied:
Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>