read_istream_filtered: propagate read error from upstream

The filter istream pulls data from an "upstream" stream,
running it through a filter function. However, we did not
properly notice when the upstream filter yielded an error,
and just returned what we had read. Instead, we should
propagate the error.

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

streaming.c

@@ -237,7 +237,7 @@ static read_method_decl(filtered)
 		if (!fs->input_finished) {
 			fs->i_end = read_istream(fs->upstream, fs->ibuf, FILTER_BUFFER);
 			if (fs->i_end < 0)
-				break;
+				return -1;
 			if (fs->i_end)
 				continue;
 		}