build: fix `-Wformat-truncation` compiler warning

The path buffer's size is `PATH_MAX`, whilst we write two paths into
it, the latter (`dent->d_name`) also having a maximum size of `PATH_MAX`,
which triggers `-Wformat-truncation` compiler warning: check the return
value of `snprintf` and mimic a system error if the path got truncated.

Needed for #7862

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build