fix(uds): guard against nil VolumeServer in handleLocate

Prevent nil pointer dereference when VolumeServer is nil by adding
an early return with UdsStatusError before accessing vs.store.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

weed/server/volume_server_uds.go

@@ -189,6 +189,11 @@ func (u *UdsServer) handleLocate(req *LocateRequest) *LocateResponse {
 		return resp
 	}
 
+	if u.vs == nil {
+		resp.Status = UdsStatusError
+		return resp
+	}
+
 	volumeId := fileId.VolumeId
 	needleId := fileId.Key
 

weed/server/volume_server_uds_test.go

@@ -162,11 +162,10 @@ func TestUdsServerProtocol(t *testing.T) {
 		t.Fatalf("Failed to read response: %v", err)
 	}
 
-	// Since vs is nil, we expect an error status (handleLocate will panic or return error)
-	// In this case, we just check we got a valid response back
+	// Since vs is nil, we expect UdsStatusError
 	status := respBuf[0]
-	if status != UdsStatusOk && status != UdsStatusNotFound && status != UdsStatusError {
-		t.Errorf("Unexpected status: %d", status)
+	if status != UdsStatusError {
+		t.Errorf("Expected UdsStatusError (%d) for nil VolumeServer, got %d", UdsStatusError, status)
 	}
 }