Initialize cache db if empty

pkg/package/database_mem.go

@@ -199,6 +199,10 @@ func (db *InMemoryDatabase) populateCaches(p Package) {
 
 	// Create extra cache between package -> []versions
 	db.Lock()
+	if db.cached == nil {
+		db.cached = map[string]interface{}{}
+	}
+
 	if _, ok := db.cached[p.GetFingerPrint()]; ok {
 		db.Unlock()
 		return