Check _kdc_db_fetch return value before dereference entry pointer

This fixes a segfault if the _kdc_db_fetch function does not find
the entry in the database (the entry pointer will be NULL if entry
is not found).

Signed-off-by: Samuel Cabrero <scabrero@zentyal.com>
Signed-off-by: Love Hörnquist Åstrand <lha@h5l.org>