Warn on missing output parameter in extract

Change-Id: Ic8dffb5527973d5c0bc57b5d193d9f98fdb7be45
diff --git a/script/korapxml2krill b/script/korapxml2krill
index 502727b..acf8d63 100644
--- a/script/korapxml2krill
+++ b/script/korapxml2krill
@@ -635,6 +635,9 @@
 # Extract XML files
 if ($cmd eq 'extract') {
 
+  # Output is required
+  pod2usage(%ERROR_HASH) unless $output;
+
   # Create new archive object
   if (-f($input[0]) && (my $archive = KorAP::XML::Archive->new($input[0]))) {