fix non error assignment to blank

Fixes  index array out of range panic when checking  blank assignments and code assigns non error to blank:
```
panic: runtime error: index out of range [recovered]
	panic: runtime error: index out of range
goroutine 156 [running]:
runtime.panic(0x671e00, 0x80a29c)
	/usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
testing.func·006()
	/usr/local/go/src/pkg/testing/testing.go:416 +0x176
runtime.panic(0x671e00, 0x80a29c)
	/usr/local/go/src/pkg/runtime/panic.c:248 +0x18d
github.com/foubarre/errcheck/lib.(*checker).Visit(0xc2098fa240, 0x7f7f2f9aab78, 0xc208fb7180, 0x0, 0x0)
	/h/g/go/src/github.com/foubarre/errcheck/lib/errcheck.go:233 +0x583
go/ast.Walk(0x7f7f2f9aa8e8, 0xc2098fa240, 0x7f7f2f9aab78, 0xc208fb7180)
	/usr/local/go/src/pkg/go/ast/walk.go:52 +0x58
go/ast.walkStmtList(0x7f7f2f9aa8e8, 0xc2098fa240, 0xc208dc9e00, 0x13, 0x20)
	/usr/local/go/src/pkg/go/ast/walk.go:32 +0xd1
go/ast.Walk(0x7f7f2f9aa8e8, 0xc2098fa240, 0x7f7f2fcdc260, 0xc2088dd1a0)
	/usr/local/go/src/pkg/go/ast/walk.go:224 +0x41e7
```