handle nil github.User in reduceUser, avoid segfault
handle nil github.User in reduceUser, avoid segfault

    panic: runtime error: invalid memory address or nil pointer dereference
    [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7cd71e]

    goroutine 21 [running]:
    github.com/Songmu/ghch.reduceUser(...)
            .../code/ghch/octokit.go:27
    github.com/Songmu/ghch.reducePR(0xc00016c300, 0xc0001a4027)
            .../code/ghch/octokit.go:21 +0x32e
    github.com/Songmu/ghch.(*Ghch).mergedPRs.func1(0xc000099600, 0xc000150000, 0x979720, 0xc000098000, 0xc00011c3c9, 0x9, 0xc00011c3d3, 0x5, 0xc00012f490, 0x7, ...)
            .../code/ghch/ghch.go:270 +0x2f9
    created by github.com/Songmu/ghch.(*Ghch).mergedPRs
            .../code/ghch/ghch.go:254 +0x2aa

Songmu/ghch