Stops attempting to get signatures for not-installed packages

This only wastes a negligible amount of time, but causes silly
log messages.

src/org/fdroid/fdroid/AppDetails.java

@@ -284,7 +284,7 @@ public class AppDetails extends ListActivity {
         // Get the signature of the installed package...
         mInstalledSignature = null;
         mInstalledSigID = null;
-        if (curver != null) {
+        if (app.installedVersion != null) {
             PackageManager pm = getBaseContext().getPackageManager();
             try {
                 PackageInfo pi = pm.getPackageInfo(appid,