Channel.c: Make sure the error code is set in _libssh2_channel_open() (#381)

File : Channel.c

Notes :
if _libssh2_channel_open() fails, set the error code.

Credit : 
mark-i-m

src/channel.c

@@ -236,6 +236,7 @@ _libssh2_channel_open(LIBSSH2_SESSION * session, const char *channel_type,
             return NULL;
         }
         else if(rc) {
+            _libssh2_error(session, rc, "Unexpected error");
             goto channel_error;
         }