Update demand.c

Fixing bug in get_category_name when category_name is NULL

src/demand.c

@@ -119,7 +119,12 @@ void set_pattern_index(list_node_t *lnode, int pattern_index)
 char *get_category_name(list_node_t *lnode)
 // Be advised: caller must free memory returned
 {
-    return strdup(get_demand_data(lnode)->category_name);
+    char *temp = get_demand_data(lnode)->category_name;
+
+    if (temp)
+        return strdup(temp);
+    else
+        return NULL;
 }
 
 void set_category_name(list_node_t *lnode, const char *category_name)