(fix) sanity check when no defaults are found (fixes #4179)

Tools/SOGoToolDumpDefaults.m

@@ -67,8 +67,8 @@
 
 - (NSString *) processDefaults: (BOOL)allDefaults
 {
-  NSUserDefaults *ud;
   NSDictionary *defaultsDict;
+  NSUserDefaults *ud;
   NSData *plistData;
 
   ud = [NSUserDefaults standardUserDefaults];
@@ -83,6 +83,9 @@
       defaultsDict = [ud persistentDomainForName: @"sogod"];
     }
 
+  if (!defaultsDict)
+    return @"No defaults found. Try to use -f.";
+
   plistData = [NSPropertyListSerialization dataFromPropertyList: (id) defaultsDict
                                          format: NSPropertyListOpenStepFormat
                                errorDescription: 0 ];