clientconf tool crashes on -add-subnets with no decoys

[ewust]

Creating a ClientConf with no decoys and trying to add phantom subnets seems to crash the tool:

./clientconf -add-subnets "1.2.3.0/24 2001::/8" -weight 1 -o test

[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x665e5c]

goroutine 1 [running]:
main.addSubnets(0xc00000e240, 0x2, 0x2, 0xc00001e400, 0xc0000121e0)
	/home/ewust/go/src/github.com/refraction-networking/gotapdance/tools/clientconf/clientconf.go:136 +0x15c
main.main()
	/home/ewust/go/src/github.com/refraction-networking/gotapdance/tools/clientconf/clientconf.go:218 +0x1214```