fix crash that happens when using an invalid log file (#706)

internal/core/core.go

@@ -86,7 +86,11 @@ func New(args []string) (*Core, bool) {
 
 	err = p.createResources(true)
 	if err != nil {
+		if p.logger != nil {
 			p.Log(logger.Error, "%s", err)
+		} else {
+			fmt.Printf("ERR: %s\n", err)
+		}
 		p.closeResources(nil, false)
 		return nil, false
 	}