Check for valid package_index when updating core index files

Fix #81

Author: cmaglie

Diff for: commands/core/update_index.go

@@ -25,6 +25,7 @@ import (
 
 	"go.bug.st/downloader"
 
+	"github.com/arduino/arduino-cli/arduino/cores/packageindex"
 	"github.com/arduino/arduino-cli/commands"
 	"github.com/arduino/arduino-cli/common/formatter"
 	"github.com/arduino/arduino-cli/configs"
@@ -84,6 +85,11 @@ func updateIndex(URL *url.URL) {
 		os.Exit(commands.ErrNetwork)
 	}
 
+	if _, err := packageindex.LoadIndex(tmp); err != nil {
+		formatter.PrintError(err, "Invalid package index in "+URL.String())
+		os.Exit(commands.ErrGeneric)
+	}
+
 	if err := indexDirPath.MkdirAll(); err != nil {
 		formatter.PrintError(err, "Can't create data directory "+indexDirPath.String())
 		os.Exit(commands.ErrGeneric)