Fix crash on launch when unexpected input was supplyed over PT protocol

Apr 13 14:32:06.000 [info] launch_managed_proxy(): Managed proxy at '/usr/bin/webtunnel-server' has spawned with PID '100'.
Apr 13 14:32:08.000 [info] managed_proxy_stderr_callback(): Managed proxy at '/usr/bin/webtunnel-server' reported via standard error: panic: assignment to entry in nil map
Apr 13 14:32:08.000 [info] managed_proxy_stderr_callback(): Managed proxy at '/usr/bin/webtunnel-server' reported via standard error: 
Apr 13 14:32:08.000 [info] managed_proxy_stderr_callback(): Managed proxy at '/usr/bin/webtunnel-server' reported via standard error: goroutine 1 [running]:
Apr 13 14:32:08.000 [info] managed_proxy_stderr_callback(): Managed proxy at '/usr/bin/webtunnel-server' reported via standard error: git.torproject.org/pluggable-transports/goptlib%2egit.Args.Add(...)
Apr 13 14:32:08.000 [info] managed_proxy_stderr_callback(): Managed proxy at '/usr/bin/webtunnel-server' reported via standard error: 	/go/pkg/mod/git.torproject.org/pluggable-transports/goptlib.git@v1.2.0/args.go:32
Apr 13 14:32:08.000 [info] managed_proxy_stderr_callback(): Managed proxy at '/usr/bin/webtunnel-server' reported via standard error: main.main()
Apr 13 14:32:08.000 [info] managed_proxy_stderr_callback(): Managed proxy at '/usr/bin/webtunnel-server' reported via standard error: 	/webtunnel/main/server/main.go:46 +0x430