Check if required fields are filled in.

src/main.cpp

@@ -43,6 +43,16 @@ int main()
     try
     {
         const entry_type entry{cgi::parse_formdata()};
+        if (entry.instance.empty() || entry.tags.empty()
+            || entry.receipts.empty() || entry.description.empty())
+        {
+            // Probably spam, since the input fields are marked as required.
+            cout << "One of the required fields (Instance, Tags, Receipts, "
+                    "Description) was not filled.\r\n"
+                 << "Please use the back button, complete the form and submit "
+                    "again.\r\n";
+            return 1;
+        }
 
         git::update_cached_repo();
         git::create_branch();