readdir.3: Handle -1 error from pathconf() in example code snippet

readdir.3: Handle -1 error from pathconf() in example code snippet

Improve the example demonstrating allocation of a buffer
for readdir_r() to handle -1 error return from pathconf().
Otherwise, naive readers may think that pathconf() return
value can be used without checking.

Reported-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>