This commit fixes the bug that stream is closed with wrong error code
(0).  This happens when STREAM or DATA frame with END_STREAM flag set
is received and it violates HTTP messaging rule (i.e., content-length
does not match) and the other side of stream has been closed.  In this
case, nghttp2_on_stream_close_callback should be called with nonzero
error code, but previously it is called with 0 (NO_ERROR).