internal/fetchdatasource: check for error earlier

Check for an error before using the other return value.

Fixes golang/go#73377.

Change-Id: Idfe6b53a179aac7f4b49ccf009150fa09bef8956
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/665638
Reviewed-by: Michael Matloob <[email protected]>
kokoro-CI: kokoro <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: jba

internal/fetchdatasource/fetchdatasource.go

@@ -230,10 +230,10 @@ func (ds *FetchDataSource) GetUnit(ctx context.Context, um *internal.UnitMeta, f
 // findUnit returns the unit with the given path in m, or nil if none.
 func (ds *FetchDataSource) findUnit(ctx context.Context, m *fetch.LazyModule, path string) (*internal.Unit, error) {
 	unit, err := m.Unit(ctx, path)
-	ds.populateUnitSubdirectories(unit, m)
 	if err != nil {
 		return nil, err
 	}
+	ds.populateUnitSubdirectories(unit, m)
 	if ds.opts.BypassLicenseCheck {
 		unit.IsRedistributable = true
 	} else {