fix hang in git fetch if pointed at a 0 length bundle

git-repo if interupted at the exact wrong time will generate zero
length bundles- literal empty files.  git-repo is wrong here, but
git fetch shouldn't effectively spin loop if pointed at a zero
length bundle.

Signed-off-by: Brian Harring <ferringb@chromium.org>
Helped-by: Johannes Sixt
Helped-by: Nguyen Thai Ngoc Duy
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bundle.c

@@ -31,8 +31,8 @@ static int strbuf_readline_fd(struct strbuf *sb, int fd)
 	while (1) {
 		char ch;
 		ssize_t len = xread(fd, &ch, 1);
-		if (len < 0)
-			return -1;
+		if (len <= 0)
+			return len;
 		strbuf_addch(sb, ch);
 		if (ch == '\n')
 			break;