unsafe use of `unsafe_load`, needs `GC.@preserve`

[rfourquet]

When using unsafe_load, we must ensure the resource being read from is kept alive, which can be accomplished via GC.@preserve.
For example, in "src/common.jl" there is

_value(r::AbstractR123{T}, i = 1, ::Type{T2} = T) where {T, T2} = unsafe_load(Ptr{T2}(pointer_from_objref(r)), i)

While highly unlikely to be practically a problem in current julia versions, a safe fix would be:

_value(r::AbstractR123{T}, i = 1, ::Type{T2} = T) where {T, T2} = GC.@preserve r unsafe_load(Ptr{T2}(pointer_from_objref(r)), i)

[sunoru]

Thanks so much for reporting this. You are right that it is unsafe. Are you willing to file a pull request for this? Or I can look into it when I get time.