Fix write after end of array on a corrupt LDB database in ldb_filter_attrs()
We narrowly avoid a security issue here because this can only happen with a duplicate attribute.
LDB will not allow these in normal conditions and until Samba 4.11 and ldb 2.0.0 LDB had other crash bugs on corrupt dataases so this isn't a security issue.
It should be backported into Samba 4.11 swiftly however.