cmd/cgo: fix panic on references to non-existing C types

cmd/cgo: fix panic on references to non-existing C types

cgo panics in Package.rewriteRef for:

var a = C.enum_test(1)
or
p := new(C.enum_test)

when the corresponding enum type is not defined.

Check nil values for Type fields and issue a proper
error instead.

Fixes #11097
Updates #12160

Change-Id: I5821d29097ef0a36076ec5273125b09846c7d832
Reviewed-on: https://go-review.googlesource.com/15264
Reviewed-by: Ian Lance Taylor <iant@golang.org>