fixed index panic error when #offsets is one less than #lines

ISSUE:
```
W0721 16:50:18.665193 10900 logs.go:355] offsets are not sorted: [%!s(uint64=1633) %!s(uint64=1499) %!s(uint64=1572)]
W0721 16:50:18.665244 10900 logs.go:357] new offsets: [1499 1500 1501]
W0721 16:50:18.668818 10900 logs.go:339] number of offsets for 0c2fa00d5aa2:/opt/zenoss/log/zenjobs.log (numLines:5 numOffsets:4) is one less than number of lines: {"message":"--- ***** ----- \n\n -------------- celery@0c2fa00d5aa2 v3.0.15 (Chiastic Slide)\n2014-07-21 16:04:24,801 INFO celery.redirected: Starting CeleryZenWorker\n -------------- celery@0c2fa00d5aa2 v3.0.15 (Chiastic Slide)","@version":"1","@timestamp":"2014-07-21T16:08:07.931Z","file":"/opt/zenoss/log/zenjobs.log","host":"0c2fa00d5aa2","offset":["2084","2101","1572","1499"],"service":"abngac0eqkphwmqlagwpy7ig9","type":"zenjobs","tags":["_grokparsefailure","multiline"],"timestamp":["2014-07-21 16:04:24,801","2014-07-21 16:04:24,801"],"loglevel":["INFO","INFO"],"logger":["celery.redirected","celery.redirected"]}
panic: runtime error: index out of range

goroutine 1 [running]:
runtime.panic(0x96f980, 0x13a2997)
	/usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
github.com/control-center/serviced/cli/api.parseLogSource(0xc210323280, 0x26f, 0x26f, 0x6e, 0x0, ...)
	/go/src/github.com/control-center/serviced/cli/api/logs.go:342 +0x10ba
github.com/control-center/serviced/cli/api.(*api).ExportLogs(0xc210095d50, 0x13c6900, 0x0, 0x0, 0xc2100d3bf0, ...)
```