Fix issue #264

Before this patch, the stream engine terminated itself
whenever it had detected an IO error. If this happened
when sending a message, the engine lost all
in-flight messages, messages waiting to be decoded,
and the last decoded message that had not been accepted,
if there was one.

The new behaviour is to terminate the engine only after
the input error has been detected and the last decoded

src/stream_engine.hpp

@@ -83,6 +83,7 @@ namespace zmq
         unsigned char *inpos;
         size_t insize;
         decoder_t decoder;
+        bool input_error;
 
         unsigned char *outpos;
         size_t outsize;