ld-discmap sets numberOfSequentialFields to -1

[Gamnn]

ld-discmap outputs numberOfSequentialFields as -1.
This breaks ld-export-metadata, and could possibly affect other tools.

Before ld-discmap:

{
  "numberOfSequentialFields": 10,
  "isSourcePal": false,
  "fsc": 3579545,
  "fieldWidth": 910,
  "sampleRate": 14318180,
  "black16bIre": 15360,
  "white16bIre": 51200,
  "fieldHeight": 263,
  "colourBurstStart": 74,
  "colourBurstEnd": 110,
  "activeVideoStart": 134,
  "activeVideoEnd": 894
}

After:

{
  "activeVideoEnd": 894,
  "activeVideoStart": 134,
  "black16bIre": 15360,
  "colourBurstEnd": 110,
  "colourBurstStart": 74,
  "fieldHeight": 263,
  "fieldWidth": 910,
  "fsc": 3579545,
  "isMapped": true,
  "isSourcePal": false,
  "numberOfSequentialFields": -1,
  "sampleRate": 14318180,
  "white16bIre": 51200
}

Tested on NTSC CAV and CLV.

[simoninns]

Fixed