Handle fpathconf() errors.

Handle fpathconf() errors.

In one place snapper calls fpathconf(.., _PC_NAME_MAX) to determine the size of
a buffer used to hold readdir_r() results.  fpathconf() may return -1 on error,
but this fpathconf() call's return value is unchecked.  This can result in
allocating a buffer that is too small for readdir_r()'s results, resulting in
out-of-bounds memory access.

Fix it by falling back to using NAME_MAX if fpathconf(.., _PC_NAME_MAX) fails.

Signed-off-by: Justin Maggard <jmaggard@netgear.com>