s4:dsdb Don't cast an ldb_val into a const char * for schema lookups

s4:dsdb Don't cast an ldb_val into a const char * for schema lookups

This removes a number of cases where we did a cast into a const char *
of an ldb_val.  While convention is to alway have an extra \0 at
data[length] in the ldb_val, this is not required, and does not occour
at least on build farm host 'svart'.

Andrew Bartlett