streaming.c: fix a memleak

streaming.c: fix a memleak

When stream_blob_to_fd() opens an input stream with a filter, the
filter gets discarded upon calling close_istream() before the
function returns in the normal case.  However, when we fail to open
the stream, we failed to discard the filter.

By discarding the filter in the failure case, give a consistent
life-time rule of the filter to the callers; otherwise the callers
need to conditionally discard the filter themselves, and this
function does not give enough hint for the caller to do so
correctly.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>