An unbuilt program doesn't have an ast::Module, so Program::AST() will just explode.
* Have a Program default to false for IsValid()
* Initialize the ast_ field to nullptr. This was previously floating.
* Return from CommonFuzzer::Run() earlier if the InputFormat is not recognised.
Fixed: chromium:1180130
Fixed: chromium:1180157
Change-Id: I9b67daa10746f386f44919a7b9ac5c171092d6e5
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/42028
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: David Neto <dneto@google.com>