x/vulndb: "no metadata for C" when computing derived symbols

[neild]

$ go run ./cmd/vulnreport fix reports/GO-2022-0417.yaml
2022/06/28 10:03:00 fix("reports/GO-2022-0417.yaml"): addExportedSymbols("github.com/containers/buildah", ""): packages.Load:
could not import C (no metadata for C)
invalid operation: cannot compare d.dh == nil (operator == not defined on untyped nil)
invalid operation: cannot compare c.ctx == nil (operator == not defined on untyped nil)
invalid operation: cannot compare key.k == nil (operator == not defined on untyped nil)
exit status 1

This is a report for the package "github.com/containers/buildah"; from the error, it looks like we have a problem with analyzing packages with a transitive C dependency.

See also #36547.

[dr2chase]

@golang/vulndb, PTAL, TYVM.