Trying to retrieve default branch segfaults

[csmith]

Running a build from master I reliably hit this shortly after startup (v0.4.7 works fine):

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x6307e8]

goroutine 1 [running]:
main.handleNotification(0x715ce0, 0xc00007c300, 0xc0000ee280, 0xc000504100, 0xc0001b2010, 0x6, 0x1c, 0xc0003be000)
	/go/src/github.com/genuinetools/ghb0t/main.go:185 +0x208
main.getNotifications(0x715ce0, 0xc00007c300, 0xc0000ee280, 0xc0001b2010, 0x6, 0x1, 0x14, 0x0, 0x0)
	/go/src/github.com/genuinetools/ghb0t/main.go:144 +0x1ab
main.main.func2(0x715ce0, 0xc00007c300, 0xc00008c010, 0x0, 0x0, 0x0, 0x0)
	/go/src/github.com/genuinetools/ghb0t/main.go:112 +0x401
github.com/genuinetools/ghb0t/vendor/github.com/genuinetools/pkg/cli.(*Program).run(0xc000104000, 0x715da0, 0xc00007ede0, 0xc00008c000, 0x2, 0x2, 0x4c15f0, 0xc00008a2a0)
	/go/src/github.com/genuinetools/ghb0t/vendor/github.com/genuinetools/pkg/cli/cli.go:179 +0x281
github.com/genuinetools/ghb0t/vendor/github.com/genuinetools/pkg/cli.(*Program).Run(0xc000104000)
	/go/src/github.com/genuinetools/ghb0t/vendor/github.com/genuinetools/pkg/cli/cli.go:89 +0x67
main.main()
	/go/src/github.com/genuinetools/ghb0t/main.go:120 +0x287

main.go:185 being:

 			defaultBranch := *notification.Repository.DefaultBranch 

Looking at the JSON returned from the /notifications endpoint by hand it looks like there isn't a default_branch key for the repository and it's not actually listed in the notification docs, so I'm not quite sure where that field is meant to come from.