decl/type_dealias: add handle the nil of decl
decl/type_dealias: add handle the nil of decl

Add handle dd is nil, such as typealias C type

Signed-off-by: Koichi Shiraishi <2e5bdfebde234ed3509bcfc18121c70b6631e207@gmail.com>

nsf/gocode,nsf/gocode,nsf/gocode,nsf/gocode