Don't use a moved-from handler:

close #1560

* Obtain the executor from the handler prior
  to transferring ownership.

CHANGELOG.md

@@ -1,3 +1,9 @@
+Version 248:
+
+* Don't use a moved-from handler
+
+--------------------------------------------------------------------------------
+
 Version 247:
 
 * Fix async_base immediate completion

include/boost/beast/core/async_base.hpp

@@ -339,8 +339,9 @@ public:
         this->before_invoke_hook();
         if(! is_continuation)
         {
+            auto const ex = get_executor();
             net::post(net::bind_executor(
-                get_executor(),
+                ex,
                 beast::bind_front_handler(
                     std::move(h_),
                     std::forward<Args>(args)...)));