Bad file descriptor on filtering empty files

In git 2.2.0 (also tested on 2.2.0.65.g9abc44b), if an external smudge/clean filter is called on an empty file git reports something like:
error: copy-fd: read returned Bad file descriptor
error: cannot feed the input to external filter cat
error: external filter cat failed

Test case:

mkdir bug
cd bug
git init
git config filter.cat.clean cat
git config filter.cat.smudge cat
echo '* filter=cat' >.gitattributes
touch a
git add a

This started in 9035d75a2be9d80d82676504d69553245017f6d4, which introduced the possible call to copy_fd in code called from apply_filter. It appears that NULL as the src argument to apply_filter is being used both as a sentinel value to indicate that the fd should be used instead and also as a representation of the contents of an empty file. I suggest switching to using fd == -1 as the sentinel as shown in the patch below.

Thanks,
Will

convert.c +1 -1

index 9a5612e..0509ac1 100644
@@ -355,7 +355,7 @@ static int filter_buffer_or_fd(int in, int out, void *data)
 
 	sigchain_push(SIGPIPE, SIG_IGN);
 
-	if (params->src) {
+	if (params->fd == -1) {
 		write_err = (write_in_full(child_process.in, params->src, params->size) < 0);
 	} else {
 		write_err = copy_fd(params->fd, child_process.in);