Panic when jaeger-tags value is set to env var without default value

[jpkrohling]

When using the --jaeger-tags flag with a value being set by env vars, the agent crashes when no default value is specified. Example:

--jaeger.tags=key=value,envVar1=${envKey1:defaultVal1},envVar2=${envKey2:defaultVal2},envVar3=${envKey3}

The Jaeger Agent will crash when parsing the value for envKey3, as it expects a default value to exist:

panic: runtime error: index out of range

goroutine 1 [running]:
github.com/jaegertracing/jaeger/cmd/agent/app/reporter.parseAgentTags(0x7ffd2b4197bb, 0x69, 0xb)
	/home/travis/gopath/src/github.com/jaegertracing/jaeger/cmd/agent/app/reporter/flags.go:72 +0x430
github.com/jaegertracing/jaeger/cmd/agent/app/reporter.(*Options).InitFromViper(0xc000127af0, 0xc0000b2900, 0x5)
	/home/travis/gopath/src/github.com/jaegertracing/jaeger/cmd/agent/app/reporter/flags.go:55 +0x9e
main.main.func1(0xc0001d0a00, 0xc00016bc20, 0x0, 0x3, 0x0, 0x0)
	/home/travis/gopath/src/github.com/jaegertracing/jaeger/cmd/agent/main.go:59 +0x121
github.com/jaegertracing/jaeger/vendor/github.com/spf13/cobra.(*Command).execute(0xc0001d0a00, 0xc000092010, 0x3, 0x3, 0xc0001d0a00, 0xc000092010)
	/home/travis/gopath/src/github.com/jaegertracing/jaeger/vendor/github.com/spf13/cobra/command.go:762 +0x465
github.com/jaegertracing/jaeger/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xc0001d0a00, 0xc0001d0a00, 0xc000127f60, 0x5)
	/home/travis/gopath/src/github.com/jaegertracing/jaeger/vendor/github.com/spf13/cobra/command.go:852 +0x2ec
github.com/jaegertracing/jaeger/vendor/github.com/spf13/cobra.(*Command).Execute(...)
	/home/travis/gopath/src/github.com/jaegertracing/jaeger/vendor/github.com/spf13/cobra/command.go:800
main.main()
	/home/travis/gopath/src/github.com/jaegertracing/jaeger/cmd/agent/main.go:101 +0x3e8

[jpkrohling]

A workaround is to simply add : to the notation, like: ${envKey3:}, instead of ${envKey3}