stream_blob_to_fd: detect errors reading from stream

We call read_istream, but never check its return value for
errors. This can lead to us looping infinitely, as we just
keep trying to write "-1" bytes (and we do not notice the
error, as we simply check that write_in_full reports the
same number of bytes we fed it, which of course is also -1).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

streaming.c

@@ -514,6 +514,8 @@ int stream_blob_to_fd(int fd, unsigned const char *sha1, struct stream_filter *f
 		ssize_t wrote, holeto;
 		ssize_t readlen = read_istream(st, buf, sizeof(buf));
 
+		if (readlen < 0)
+			goto close_and_exit;
 		if (!readlen)
 			break;
 		if (can_seek && sizeof(buf) == readlen) {