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>
4 files changed
tree: 461f0301e83d2d6acdbdf4aff283e647bba2004f
  1. api/
  2. doc/
  3. lib/
  4. misc/
  5. src/
  6. test/
  7. .gitattributes
  8. .gitignore
  9. AUTHORS
  10. CONTRIBUTING.md
  11. CONTRIBUTORS
  12. favicon.ico
  13. LICENSE
  14. PATENTS
  15. README.md
  16. robots.txt
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