Segfault related to timestamp formatting

[shazow]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x76588a]
goroutine 6328 [running]:
github.com/shazow/ssh-chat/chat/message.Theme.Timestamp(...)
        /home/shazow/projects/ssh-chat/chat/message/theme.go:168
github.com/shazow/ssh-chat/chat/message.(*User).render(0xc0002f5680, 0x9667e0, 0xc00032aff0, 0xa5c65e126f9c, 0xc84f80)
        /home/shazow/projects/ssh-chat/chat/message/user.go:185 +0x34a
github.com/shazow/ssh-chat/chat/message.(*User).writeMsg(0xc0002f5680, 0x9667e0, 0xc00032aff0, 0xc84f80, 0x0)
        /home/shazow/projects/ssh-chat/chat/message/user.go:193 +0x49
github.com/shazow/ssh-chat/chat/message.(*User).HandleMsg(0xc0002f5680, 0x9667e0, 0xc00032aff0, 0x1, 0x1)
        /home/shazow/projects/ssh-chat/chat/message/user.go:207 +0xb0
github.com/shazow/ssh-chat/chat/message.(*User).Consume(0xc0002f5680)
        /home/shazow/projects/ssh-chat/chat/message/user.go:125 +0x4c
created by github.com/shazow/ssh-chat.(*Host).Connect
        /home/shazow/projects/ssh-chat/host.go:99 +0x165