iostream: protect write side against bad custom IO interfaces too

This is the same as the previous commit, just for the write side error
handling.

If we have a bad custom IO low-level driver that returns
DC_STATUS_SUCCESS with zero bytes written, and we have a write() user
that isn't ready for partial writes, we just consider that an IO error.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

src/iostream.c

@@ -260,6 +260,9 @@ dc_iostream_write (dc_iostream_t *iostream, const void *data, size_t size, size_
 		if (status != DC_STATUS_SUCCESS)
 			return status;
 
+		if (!nbytes)
+			return DC_STATUS_IO;
+
 		data = (void *)(nbytes + (char *)data);
 		size -= nbytes;
 	}