Fix simple java error in LIBRARY_INDEX_URL_GZ assignment

Being a final variable it must be assigned only once.

Author: cmaglie

Diff for: arduino-core/src/cc/arduino/Constants.java

@@ -80,10 +80,11 @@ public class Constants {
     String externalLibraryIndexUrl = System.getProperty("LIBRARY_INDEX_URL");
     if (externalLibraryIndexUrl != null && !"".equals(externalLibraryIndexUrl)) {
       LIBRARY_INDEX_URL = externalLibraryIndexUrl;
-      LIBRARY_INDEX_URL_GZ = "";
       String externalLibraryIndexUrlGz = System.getProperty("LIBRARY_INDEX_URL_GZ");
       if (externalLibraryIndexUrlGz != null && !"".equals(externalLibraryIndexUrlGz)) {
         LIBRARY_INDEX_URL_GZ = externalLibraryIndexUrlGz;
+      } else {
+        LIBRARY_INDEX_URL_GZ = "";
       }
     } else {
       LIBRARY_INDEX_URL = "http://downloads.arduino.cc/libraries/library_index.json";