Check the info-hash target in p.call_target

Without it an unknown info-hash segfaults the process.

src/command_download.cc

@@ -431,6 +431,9 @@ p_call_target(const torrent::Object::list_type& args) {
   const std::string& peer_id = itr++->as_string();
   const std::string& command_key = itr++->as_string();
 
+  if (download == nullptr)
+    throw torrent::input_error("invalid parameters: info-hash not found");
+
   torrent::HashString hash;
 
   if (peer_id.size() != 40)