t: handle EOF in test_copy_bytes()

The test_copy_bytes() function claims to read up to N bytes,
or until it gets EOF. But we never handle EOF in our loop,
and a short input will cause perl to go into an infinite
loop of read() getting zero bytes.

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

t/test-lib-functions.sh

@@ -999,6 +999,7 @@ test_copy_bytes () {
 			my $s;
 			my $nread = sysread(STDIN, $s, $len);
 			die "cannot read: $!" unless defined($nread);
+			last unless $nread;
 			print $s;
 			$len -= $nread;
 		}