gopls/internal/golang: fix crash in package references

Fix a crash found via telemetry, which may be encountered when finding
references for a package where one of the package files lacks a valid
name.

Fixes golang/go#66250

Change-Id: Ifca9b6b7ab2ab8181b70e97d343fa2b072e866eb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/570676
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/golang/references.go

@@ -198,11 +198,14 @@ func packageReferences(ctx context.Context, snapshot *cache.Snapshot, uri protoc
 		if err != nil {
 			return nil, err
 		}
-		refs = append(refs, reference{
-			isDeclaration: true, // (one of many)
-			location:      mustLocation(f, f.File.Name),
-			pkgPath:       widest.PkgPath,
-		})
+		// golang/go#66250: don't crash if the package file lacks a name.
+		if f.File.Name.Pos().IsValid() {
+			refs = append(refs, reference{
+				isDeclaration: true, // (one of many)
+				location:      mustLocation(f, f.File.Name),
+				pkgPath:       widest.PkgPath,
+			})
+		}
 	}
 
 	return refs, nil

gopls/internal/test/marker/testdata/fixedbugs/issue66250.txt

@@ -0,0 +1,17 @@
+This bug checks the fix for golang/go#66250. Package references should not
+crash when one package file lacks a package name.
+
+TODO(rfindley): the -ignore_extra_diags flag is only necessary because of
+problems matching diagnostics in the broken file, likely due to poor parser
+recovery.
+
+-- flags --
+-ignore_extra_diags
+
+-- a.go --
+package x //@refs("x", "x")
+
+-- b.go --
+
+func _() {
+}