Handle invalid deflate frames:

This fixes a problem where an assert is generated or an
error is ignored when an invalid deflate stream is produced
after appending the final empty deflate block.

test/beast/websocket/error.cpp

@@ -41,6 +41,7 @@ public:
         check("boost.beast.websocket", error::failed);
         check("boost.beast.websocket", error::handshake_failed);
         check("boost.beast.websocket", error::buffer_overflow);
+        check("boost.beast.websocket", error::partial_deflate_block);
     }
 };