Fix vis family of functions to not leak

Fix vis family of functions to not leak

The code uses an internal helper function to avoid code repetition. But
to get there, the function takes a pointer to a pointer, so that the few
functions that require returning an allocated buffer can get hold of it
this way.

The problem is that the user might pass a NULL pointer and trigger an
internal allocation even if the functions are not expected to do so.

Add a new internal helper for non-allocations, that will assert that
condition, and make any other function that requires this behavior call
this one instead.

Warned-by: coverity