applied patch from Stefan Kost Daniel

* xmlcatalog.c: applied patch from Stefan Kost
Daniel
diff --git a/xmlcatalog.c b/xmlcatalog.c
index 6e614ce..f974d3a 100644
--- a/xmlcatalog.c
+++ b/xmlcatalog.c
@@ -38,7 +38,7 @@
 static int del = 0;
 static int convert = 0;
 static int verbose = 0;
-static char *filename;
+static char *filename = NULL;
 
 
 #ifndef XML_SGML_DEFAULT_CATALOG
@@ -556,7 +556,7 @@
 	}
     }
     if ((!sgml) && ((add) || (del) || (create) || (convert))) {
-	if (noout) {
+	if (noout && filename && *filename) {
 	    FILE *out;
 
 	    out = fopen(filename, "w");