internal/worker: check modulePath arg to FetchAndUpdateState

Check that the module path is not internal.UnknownModulePath.

Change-Id: I3a204f27a1d82becec8fbdb95290ccc00b308ccb
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/304389
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/internal/worker/fetch.go b/internal/worker/fetch.go
index 5035dda..e7ca98d 100644
--- a/internal/worker/fetch.go
+++ b/internal/worker/fetch.go
@@ -59,6 +59,9 @@
 	if !utf8.ValidString(modulePath) {
 		log.Errorf(ctx, "module path %q is not valid UTF-8", modulePath)
 	}
+	if modulePath == internal.UnknownModulePath {
+		return http.StatusInternalServerError, "", errors.New("called with internal.UnknownModulePath")
+	}
 	if !utf8.ValidString(requestedVersion) {
 		log.Errorf(ctx, "requested version %q is not valid UTF-8", requestedVersion)
 	}