Fix a segfault with input/output modules.

Fix a segfault with input/output modules.

Some functions in std.c were using

  const char *prefix = sdi->driver->name;

but were called from input/output modules as well (which don't have
a "driver" field).

As a temporary workaround, use "unknown" as prefix in such cases until
a more permanent solution is implemented.

This fixes bug #813.