gopls/internal/cache: guard against malformed paths in port.matches

It's possible that we may encounter the path inconsistency of
golang/go#67288 due to unclean or relative paths. Guard against this
with a new bug report.

For golang/go#67288

Change-Id: I37ac1f74334bcb9e955d75e436f74398c73f0acb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/626015
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/cache/port.go

@@ -141,6 +141,11 @@ var (
 func (p port) matches(path string, content []byte) bool {
 	ctxt := build.Default // make a copy
 	ctxt.UseAllFiles = false
+	path = filepath.Clean(path)
+	if !filepath.IsAbs(path) {
+		bug.Reportf("non-abs file path %q", path)
+		return false // fail closed
+	}
 	dir, name := filepath.Split(path)
 
 	// The only virtualized operation called by MatchFile is OpenFile.