internal/licenses: return zero value if Cover fails
internal/licenses: return zero value if Cover fails

If licensecheck.Checker.Cover's second return value is false,
its first can have a Percent of NaN. This breaks JSON serialization
in DB.saveVersion.

Return the zero value in this case.

Change-Id: I2b6ac30c9a7ed28e0b725c3508ba4b8e841768b1
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/666962
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite