Panics when run against generated file

[dmac]

Normally golint ignores generated files, but it panics when all files passed to it are generated.

Minimal repro:

$ echo '// Code generated by foo. DO NOT EDIT.
quote>
quote> package foo
quote> ' > foo.go
$ golint foo.go
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x11d3296]

goroutine 1 [running]:
github.com/golang/lint.(*pkg).typeCheck(0xc420013860, 0xc4200c0000, 0xc4200190b0)
	/Users/dmac/go/src/github.com/golang/lint/lint.go:282 +0x366
github.com/golang/lint.(*pkg).lint(0xc420013860, 0x35, 0x235, 0x1)
	/Users/dmac/go/src/github.com/golang/lint/lint.go:152 +0x40
github.com/golang/lint.(*Linter).LintFiles(0xc420047b60, 0xc420047c00, 0xc420047bc0, 0xc420047db8, 0x235, 0x0, 0x0)
	/Users/dmac/go/src/github.com/golang/lint/lint.go:114 +0x71c
main.lintFiles(0xc420014860, 0x1, 0x1)
	/Users/dmac/go/src/github.com/golang/lint/golint/golint.go:114 +0x265
main.main()
	/Users/dmac/go/src/github.com/golang/lint/golint/golint.go:78 +0x871

[cespare]

/cc @dsnet

[cespare]

I think #299 is a fix for this.