dj(1): add a ton of assertions, fix if statement, fix io[i] mixups

src/dj.c

@@ -16,6 +16,7 @@
  * along with this program. If not, see https://www.gnu.org/licenses/.
  */
 
+#include <assert.h> /* assert(3) */
 #include <errno.h> /* errno */
 #include <fcntl.h> /* open(2) */
 #include <stdio.h> /* fprintf(3), stderr */
@@ -68,14 +69,18 @@ static int write_flags = O_WRONLY | O_CREAT;
 static void *
 Io_bufalloc(struct Io *io){
 
-	return (io->buf = malloc(io->bs * (sizeof *io->buf)));
+	return io != NULL
+		? (io->buf = malloc(io->bs * (sizeof *io->buf)))
+		: NULL;
 }
 
 /* Fills the unused portion of io's buffer with padding, updating io->bufuse.
  * Returns io. */
 static struct Io *
 Io_bufrpad(struct Io *io, int padding){
-	
+
+	assert(io != NULL);
+
 	memset(io->buf + io->bufuse, padding, io->bs - io->bufuse);
 	io->bufuse = io->bs;