Fix git-fast-export for zero-sized blobs

Writing 1 elements of size 0-byte successfully will cause fwrite(3) to
return 0, and flagging it as error is a mistake.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fast-export.c

@@ -103,7 +103,7 @@ static void handle_object(const unsigned char *sha1)
 	mark_object(object);
 
 	printf("blob\nmark :%d\ndata %lu\n", last_idnum, size);
-	if (fwrite(buf, size, 1, stdout) != 1)
+	if (size && fwrite(buf, size, 1, stdout) != 1)
 		die ("Could not write blob %s", sha1_to_hex(sha1));
 	printf("\n");