Skip the length sanity check if d_name is pointer or less.

Skip the length sanity check if d_name is pointer or less.

[perl #127511] v5.23.7-308-g1d41bb7 broke t/op/threads-dirh.t on solaris threaded builds

In other words, skip it if the dirent->d_name is a pointer (char *)
or less (struct-final char d_name[1], as it seems to be in Solaris).

The length sanity check is meant for places where the d_name is
a true array.

Follow-up to 1d41bb72.