Uncaught TypeError when loading trkseg with only one trkpt

```
leaflet-omnivore.js:1019 Uncaught TypeError: Cannot read property 'line' of undefined
at getTrack (leaflet-omnivore.js:1019)
at Object.gpx (leaflet-omnivore.js:991)
at gpxParse (leaflet-omnivore.js:213)
at XMLHttpRequest.onload (leaflet-omnivore.js:114)
at XMLHttpRequest.loaded (leaflet-omnivore.js:395)
```
after omnivore.gpx("x.gpx")

The file contains a _trkseq_ with only only one _trkpnt_. Unfortunately this problem stops reading all other segments too.

```

Name

a


628.00
2011-09-10T10:59:03Z


627.50
2011-09-10T10:59:06Z




626.50
2011-09-10T10:59:10Z




626.00
2011-09-10T10:59:11Z


626.50
2011-09-10T10:59:13Z

```

Start in leaflet-omnivore.js at getTrack around line 1019, called through gpx. Reproduce with the supplied GPX containing a single-point track segment and verify how later segments are handled. Done means loading the file no longer throws an uncaught TypeError and the remaining segments are still parsed.

Written by the indexing model from the issue text.