Sketchy useage of `unsafe_wrap`/`unsafe_string`

[oscardssmith]

This library makes heavy use of unsafe_wrap and unsafe_string, (see https://github.com/wildart/LMDB.jl/blob/bdc2df64debb30ffc4f43ae0c92107d35de0c8af/src/common.jl#L13) 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

[vtjnash]

The object's memory still isn't GC-rooted, due to the illegal call to pointer here

LMDB.jl/src/common.jl

Line 5 in a1e9607

MDBValue(val::String) = MDB_val(Csize_t(sizeof(val)), convert(Ptr{Cvoid},pointer(val)))

[maleadt]

Yeah I'm moving too fast here. I had this fixed in a local branch, and wanted to split things up into logical PRs, but failed to do so properly. Folding the better fix into #52