weed mini builds MasterOptions directly and never set metricsHttpPort, so
reading it in toMasterOption dereferenced nil and crashed startup. Point
mini's master, volume and filer at its single -metricsPort listener, the
same way weed server does, and treat an unset port as disabled so a
partially initialised MasterOptions cannot panic again.
Reproduced with 'weed mini -dir=... -s3.port=...', which is what the S3
filer-group and delete-regression suites start.
Chris Lu
2f26d5779b