Sketchy useage of unsafe_wrap/unsafe_string

[oscardssmith]

This library makes heavy use of unsafe_wrap and unsafe_string, (see

LMDB.jl/src/common.jl

Line 13 in bdc2df6

function _mbd_unpack(::Type{T}, mdb_val::MDB_val) where {T <: String}

) but doesn't seem to ensure that they aren't used on data that comes from Julia. The intention of these functions is that they are used on foreign memory only (since otherwise, you will get a double free when the original and wrapped version both get freed).

[vtjnash]

The issue isn't that (per se) but rather that the call to pointer in the constructor likely destroys the GC information, leading to use after free bugs later