This fixes an oversight from 5c995139.  This has no actual consequence as
PG_TEMP_FILE_PREFIX and PG_TEMP_FILES_DIR have the same value so when
bumping on a temporary path the directory scan was still moving on to
the next entry instead of skipping the rest of the scan, but let's keep
the logic correct.

Author: Michael Banck
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20190314.115417.58230569.horiguchi.kyotaro@lab.ntt.co.jp
Backpatch-through: 11