Making sure the callback is set before calling it.

Author: deanberris

boost/network/protocol/http/client/connection/async_normal.hpp

@@ -172,7 +172,7 @@ namespace boost { namespace network { namespace http { namespace impl {
         } else {
           set_errors(ec ? ec : boost::asio::error::host_not_found);
           boost::iterator_range<const char*> range;
-          callback(range,ec);
+          if (callback) callback(range,ec);
         }
       }
     }