HPCC-8677 Fix setPasswordsFromSDS to avoid memory leaks

When an ESP is started, memory leaks are reported due to multiple
calls to setPasswordsFromSDS(). That is because a passwordProvider
pointer is created (new) every time when setPasswordsFromSDS() is 
called.  In this fix, the passwordProvider is initialized to NULL and it will 
be created only if the setPasswordsFromSDS() is called at the first time
(by checking the passwordProvider).

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>