openldap: limit max incoming size
Set the maximum allowed size of an incoming LDAP message, which to
OpenLDAP means that it allows malloc() up to this size. If not set,
there is no limit and we instead risk a malloc() failure.
The limit is arbitrarily set to 256K as I can't figure out what a
reasonable value should be.
OpenLDAP docs: https://openldap.org/software/man.cgi?query=lber-sockbuf&apropos=0&sektion=0&manpath=OpenLDAP+2.6-Release&arch=default&format=html
Bug: https://issues.oss-fuzz.com/issues/
432441303
Closes #19087