internal/gcimporter: add a missing return if objectpath fails

The export data encoding was not symmetric in the case where objectpath
fails.

Fixes golang/go#61670

Change-Id: Ifaaa8eaed7ee5e28aa3b5ee7e9600b9e379e1968
Reviewed-on: https://go-review.googlesource.com/c/tools/+/514355
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Robert Findley <rfindley@google.com>

internal/gcimporter/iexport.go

@@ -929,10 +929,11 @@ func (w *exportWriter) objectPath(obj types.Object) {
 		// that export/import is producing a correct package.
 		//
 		// TODO: remove reportf once we have such confidence.
-		objectPath = ""
+		w.string("")
 		if w.p.reportf != nil {
 			w.p.reportf("unable to encode object %q in package %q: %v", obj.Name(), obj.Pkg().Path(), err)
 		}
+		return
 	}
 	w.string(string(objectPath))
 	w.pkg(obj.Pkg())