HPCC-10310 ESP cores and restarts when invalid credentials are presented

Running Fedora, calling ldap_get_option(LDAP_OPT_ERROR_STRING) after submitting
invalid credentials results in a NULL string which is getting passed to strstr
resulting in a core in ESP. This change first checks if there is a valid string
before calling strstr

Signed-off-by: William Whitehead <william.whitehead@lexisnexis.com>
Merge pull request #5078 from RussWhitehead/espCore

HPCC-10310 ESP cores and restarts when invalid credentials are presented

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>