fixed a problem with length limited ldap values

fixed a problem with length limited ldap values

The core ldb code for string matching assumed NULL terminated strings,
whereas the anr module used data_blob_const() to effectively truncate
a ldb_val by changing its length. The ldb code is supposed to be based
around length limited blobs, not NULL terminated strings, so the
correct fix was to change the string comparison functions to be length
limited